This is the big question and only you an answer it. What do you want? How big do you need? Buses come in front engine and rear engine, flat front or conventional, gas and diesel, and lengths up to 40 feet long.
Do you want to full time RV in it, weekends only, Tiny house that gets parked in one spot and stays?
So, I am working on 2 buses right now. One for my wife and I, one for my daughter. Ours is a vacation RV, weekends, long weeks traveling, but not full time. We own a decent house we can afford.
My daughter just got a bus to convert into a tiny house. She plans to park it on some land and live in it full time. Our needs are different.
Hers has a 366 gas engine and a AT545 trans, it will be parked, so who cares. Mine has a DT-466 with MT643, I will be driving mine.
I paid $1500 for my 1985 66 Passenger Blue Bird International, she paid $825 for her 1991 GMC Ward 66 passenger. Both are about 38 feet long and both conventional style. But, different uses.
You need to figure out you final plans for the bus and then see what works for you. I like front engine buses, some like rear engine. Its your bus, you tell us what you want and we can better help you.
Where to buy? I got one off of Craigslist from a Church north of Seattle Wa. The other I got off of Public Surplus Auction site out of Idaho. You can get from dealers too, but will pay more.
Let us know how we can help