Upvote:1
75USD per night should get you very comfortable accommodation. 50USD per day for meals should be sufficient, but will depend on what kind of restaurants you'll want to frequent.
Bangkok is quite a hub. Is that not the obvious primary destination for your trip?
Public transport is quite good, but perhaps not so easy to navigate for foreigners. Renting a car will make your life easy, but is not so common. Renting scooters is much more common and cheap.
Visiting Myanmar would be interesting, but much more a hassle than visiting Laos or Vietnam. Laos is safe, but indeed less touristy compared to Vietnam.
Based on what you're describing for your trip, you don't need a local guide.