Should I buy a prepaid Sunpass when travelling by car around Florida?

The surcharges imposed by some rental car companies are not astronomical like Choster's referenced Bloomberg article. I recently (August) used a rental car (Enterprise) in Florida for a trip from north Florida to Miami and passed several toll-by-plate stations. The cost was $3 plus the toll, not $20 as claimed in Bloomberg.

I paid cash at the remainder of the toll plazas, which was quick and easy since the majority of vehicles on the road use the SunPass, so there was never more than a car ahead of me.

Depending on where you are staying, going to and from the Orlando parks may not require toll roads. There are plenty of secondary highways, albeit a bit slower due to traffic lights and such, but sounds like you are on vacation, so no need for speed ;-)

You are not required to obtain or use a SunPass transponder to travel on Florida's Turnpike, but it may simplify your life. Of course, if you forego it, you should avoid lanes, booths, or roads indicated as SunPass Only, which will incur a toll-by-plate charge in addition to the toll— and to which your rental car company will add a significant surcharge.

As to the total amount, you can use the Toll and Mileage Calculator provided by the Turnpike operator. While you don't specify what your exact starting and end points are, a trip on the Turnpike from Golden Glades to Interstate 4 returns a total cash price of $17.97 each way, for which SunPass would offer a discount of $3.64. At that rate, it does not seem you would recoup the $13 SunPass rental fee, but it might still be "worth it" to you for the convenience and time savings.

Only toll plazas on the mainline of the Florida Turnpike (mile markers 49 to 309) and at Golden Glades at the start of the access spur are consistently staffed. The Paying Tolls page from the operator states

Toll plazas along the Mainline feature at least one lane staffed by an attendant 24 hours per day. If you require change, receipts or even directions, look for an overhead green sign. If you have exact change, proceed toward the lanes featuring overhead blue signs.

and later

Not all exit and entrance ramp toll plazas are staffed, so motorists should be prepared to have exact change at these interchanges, especially after 7 p.m. or on weekends. Toll collection baskets do not make change and drivers should not attempt to insert paper currency in the machinery. Quarters are preferred, but coin baskets accept any combination of nickels, dimes and quarters. You may purchase a roll of quarters from an attendant at any toll plaza along the Mainline. The collection baskets do not accept credit or debit cards and inserting them in the machine may result in the loss of or damage to the card.

There are a few service plazas along the route offering food and gasoline, also noted on the exit list, so it is not necessary to exit the Turnpike for such services.

Also applicable for this trip (mile markers added):

While the northern and southern sections of Florida's Turnpike employ the coin system of toll collection, the rural area in between uses the ticket system. Motorists traveling between Three Lakes in Osceola County [mile 236] and Lantana in Palm Beach County [mile 88] will receive an electronically-created toll card showing their point of origin and time they traveled through the lane. This card is not a SunPass and you shouldn't use it to travel through SunPass-only lanes, as you'll receive a violation notice.

Furthermore, only electronic billing is available on the southernmost part of the Turnpike (the Homestead Extension/Palmetto Expressway), and so you will need to take the non-Express lanes of Interstate 95 or another route a distance north of the city to the toll plaza at Golden Glades, mile marker 0 on the access spur. Exit 255 in the Orlando area, lastly, is SunPass-only.

Google Maps suggests you can avoid tolls altogether by taking the non-Express lanes of Interstate 95 north to U.S. 192 or Florida Route 528/520 (the non-tolled portion of 528), but the route is longer and more susceptible to traffic delays.