What kind of life is required for practicing Metta?

Perception of metta developed even for as long as it takes to pull a cow's udder is of great fruit.

Staying at Savatthi. "Monks, if someone were to give a gift of one hundred serving dishes [of food] in the morning, one hundred at mid-day, and one hundred in the evening; and another person were to develop a mind of good-will — even for the time it takes to pull on a cow's udder — in the morning, again at mid-day, and again in the evening, this [the second action] would be more fruitful than that [the first].

"Thus you should train yourselves: 'Our awareness-release through good-will will be cultivated, developed, pursued, handed the reins and taken as a basis, given a grounding, steadied, consolidated, & well-undertaken. That's how you should train yourselves." (Sn20.4)

If with uncorrupted mind you feel good will for even one being, you become skilled from that. But a Noble One produces a mind of sympathy for all beings, an abundance of merit. (Can't remember where this is from)

There is a range of conditions required for development such as a human or otherwise suitable birth, desire and effort.

There are various circumstances of beings and some are more conducive than others to development.

Monkhood doesn't guarantee fewness of wishes & projects, nor a conducive environment; it does not guarantee being surrounded by nor being taught right view.

If one wants to develop the mind, is bent on doing it, is willing to hand it reins, even if it means homelessness and poverty; it would be pretty much impossible to stop one from winning.

If one is looking or waiting for favorable conditions, for a good teacher, for the approval from these or those people; one is bent on procastinating and making excuses ala 'i am too busy, i am too stressed, i am too tired, i am too disheartened, i have too much of this or not enough of that etc etc.

Ideally one would have all the blessings and the best circumstances but these things are hard to obtain and are subject to change. Therefore i think one should make most of what is at hand and try to simplify one's livelihood if there is any opening.

Look no further than Theranama Sutta to see that training solo is completely fine. As for livelihood (a job or smthn) it isn't necessarily a disqualifying factor and there is rìght livelihood.

Metta is to be friendly in mind and in action. Metta meditation help develop this mentally by:

• breaking distinction who you would teat certain people and / groups of people
• develop qualities of friendship in mind which can later manifest as actions

OP: What kind of life is required for practicing Metta?

You can be at any point when you start Metta. But as you develop one should develop the quality of friendship. This does not mean one should associate with everyone but maintaining proper distance one can develop the qualities in the following quotes:

(1) He is pleasant [loving]. piyo ca hoti

(2) He is agreeable. manāpo ca

(3) He is respectful. garu ca

(4) He is worthy of respect. bhāvanīyo ca

(5) He admonishes, patient when admonished. vattā ca vacana-k,khamo ca

(6) He is profound in conversation. gambhīrañ ca kathaṁ kattā hoti,

(7) He does not engage us in the impossible. no ca aṭṭhāne niyojeti.

(Piya) Mitta Sutta 2

(1) He gives what is difficult to give. duddadaṁ dadāti

(2) He does what is difficult to do. dukkaraṁ karoti

(3) He tolerates what is difficult to tolerate. dukkhamaṁ khamati

(4) His secrets he reveals. guyham assa āvi karoti

(5) Your secrets he keeps. guyham assa pariguhati

(6) In misfortune he forsakes you not. āpadāsu na jahati

(7) He does not look down (on you) when you have failed [are destitute]. khīṇena nâtimaññati

(Piya) Mitta Sutta 1

“Young houselord, there are these 4 who should be known as true-hearted friends:

(1) The helper (upakara)

should be known as a true-hearted friend.

(2) The one constant in joy and in sorrow (samana,sukha,dukkha)

should be known as a true-hearted friend.

(3) The good counsellor (atth’akkhayi)

should be known as a true-hearted friend.

(4) The caring (anukampaka)

should be known as a true-hearted friend.

(1) The helper, young houselord, should be known as a true-hearted friend for these four reasons:

(a) He guards you when you are heedless. pamattaṁ rakkhati

(b) He guards your property when you are heedless. pamattassa sāpateyyaṁ rakkhati

(c) He is a refuge to you when you are in fear [in danger]. bhītassa saraṇaṁ hoti

(d) When you are faced with tasks, he provides you with double the supplies you need. uppannesu kicca,karaṇīyesu tad di,guṇaṁ bhogaṁ anuppadeti

The helper, young houselord, should be known as a true-hearted friend for these four reasons.

(2) The one constant in joy and in sorrow [a friend through thick and thin] should be known as a true-hearted friend for these 4 reasons:

(a) He tells you his secrets. guyham assa ācikkhati

(b) He keeps your secrets. guyham assa parigūhati

(c) He does not abandon you in your troubles. āpadāsu na vijahati

(d) He would even give up his life for you. jīvitaṁ pi’ssa atthāya pariccattaṁ hoti

The one who is constant in joy and sorrow, young houselord, should be known as a true-hearted friend for these four reasons.

(3) The good counsellor [the one who shows benefits or the goal] should be known as a truehearted friend for these four reasons:92

(a) He restrains you from bad. pāpā nivāreti

(b) He exhorts you to do good. kalyāṇe niveseti

(c) He lets you hear what you have not heard before. assutaṁ sāveti

(d) He shows you the way to heaven. saggassa maggaṁ ācikkhati

The good counsellor, young houselord, should be known as a true-hearted friend for these four reasons.

(4) The caring should be known as a true-hearted friend for these four reasons:

(a) He does not delight in your misfortunes. abhaven’assa na nandati

(b) He delights in your good fortunes. bhavena’ssa nandati

(c) He prevents others from speaking ill of you. avaṇṇaṁ bhaṇamānaṁ nivāreti

(d) He commends others who speak well of you. vaṇṇaṁ bhaṇamānaṁ pasaṁsati

The one who is caring, young houselord, should be known as a true-hearted friend for these 4 reasons.”

The Blessed One said this.

Having said that, the Sugata [the well-farer], the Teacher, further said this:

The friend who is a helper, and the friend in weal and woe,

and the friend who gives good counsel, and the caring friend—

these four are (true) friends, thus the wise should know

and devotes himself with respect as a child would his parents

Sigal’ovada Sutta

At a spiritual level one should try to develop these qualities:

(1) The accomplishment of faith (saddhā,sampadā),

(2) The accomplishment of moral virtue (sīla,sampadā),

(3) The accomplishment of charity (cāga,sampadā),

(4) The accomplishment of wisdom (paññā,sampadā)

Dīgha,janu Sutta

OP: So one should be content and easy to support, unburdened with duties and living lightly.

This is not limited to those who want to practice Metta. This is a general list of qualities for those who want to be liberated.

Content:- One should be content in 12 way, i.e., concent in 3 ways (based on what is received, based on one's strengths and what is benefiting) with regard to the 4 requisites (robes, alms, logging and medicine). E.g. if one receives a robe which is too heavy one will exchange it to a lighter one with a fellow monastic.

Easy to support:- When one is content one is easy to support.

Few duties:- one should not burden oneself with undue responsibilities and activities. One must discharge due responsibilities and activities.

Living lightly:- one equanimous towards Eight Worldly Conditions and unburdened.

OP: Is Metta best practiced as a monk?

Metta can be practised by anyone. Above passage list qualities of would who want to develop spiritually. These qualities can be adopted to a householder's life too.

OP: How can one practice Metta in this [predominantly] materialistic world?

This is by being friendly in mind and in action, without hurting or compromising oneself. In the material world one must ensure there is some distance so one is not hurt by bad friends:

“Young householder, there are these four who should be known as enemies in the guise of friends:

(1) The downright taker,

young householder, should be known as an enemy in the guise of a friend.

(2) The mere talker,

young householder, should be known as an enemy in the guise of a friend.

(3) The sweet-talker, young householder, should be known as enemy in the guise of a friend.

(4) The destructive companion,

young householder, should be known as an enemy in the guise of a friend.

Young householder, these are the four who should be known as enemies in the guise of friends.

(1) The downright taker, young houselord, should be known as an enemy in the guise of a friend for these four reasons:

(a) He only takes (without ever giving). añña-d-atthu,haro hoti

(b) With little, he desires much. appena bahum icchati

(c) He does what needs doing only out of fear. bhayassa kiccaṁ karoti

(d) He serves with an ulterior motive. sevati attha,kāraṇā

The downright taker, young houselord, should be known as an enemy in the guise of a friend for these four reasons.

(2) The mere talker, young houselord, should be known as an enemy in the guise of a friend for these four reasons:

(a) He claims to having been hospitable in the past. atītena paṭisantharati

(b) He promises hospitality in the future. anāgatena paṭisantharati

(c) He worms his way with empty favours. niratthakena saṅgaṇhāti

(d) In present tasks, he feigns misfortune. paccuppannesu kiccesu vyasanaṁ dasseti

For these four reasons, young houselord, the mere talker should be known as an enemy in the guise of a friend.

(3) The sweet talker, young houselord, should be known as an enemy in the guise of a friend for these four reasons:

(a) He approves of your doing wrong. pāpakaṁ pi’ssa anujānāti

(b) He approves of your doing right [or disapproves of it, depending on what profits him]. kalyāṇaṁ pi’ssa anujānāti

(c) He sings your praises to your face. sammukhāssa vaṇṇaṁ bhāsati

(d) He runs you down behind your back. param,mukhāssa avaṇṇaṁ bhāsati

For these four reasons, young houselord, the sweet talker should be known as an enemy in the guise of a friend.

(4) The destructive companion, young houselord, should be known as an enemy in the guise of a friend for these four reasons:

(a) He is your companion in taking strong drinks, distilled drinks, fermented drinks that which causes heedlessness.

(b) He is your companion in roaming (and loitering in) the streets at unseemly hours.

(c) He is your companion in frequenting fairs [shows].

(d) He is your companion in indulging in gambling, a basis for heedlessness.

For these four reasons, young houselord, the destructive companion should be known as an enemy in the guise of a friend.”

The Blessed One said this.

Having said that, the Sugata [the well-farer], the Teacher, further said this:

The friend who merely takes, and the friend who only talks,

and the one who sweet-talks, and the companion who brings destruction—

these four are not friends. Thus the wise should know:

they should be avoided from afar like a dangerous path.

Sigal’ovada Sutta

"content & easy to support" is Santussako ca subharo ca.

There's a fairly long description of that, with sutta references, in this definition of asantuṭṭhitā.

I think that "easy to support" is one of the things that the Buddha said that monks ought to be.

It's a formula, a phrase that's used in several suttas (I imagine it's in the Vinaya too).

The commentary says that the Karaniya Metta Sutta was addressed to a group of all/only monks.