||There are pros and cons on the type of bus you choose. I am quite new at all this, but may be able to help you. (see my post here...GMC - RTS, below...
School busses are really cheap, find a local operator with a fleet, or a school system getting rid of them, and you may find one as cheap as $500-$1000. These busses are simple in design. Easy to work on, service, maintain, but they are just ugly as a bag of rocks! (No offense to you school bus owners out there! HA!)
Transit busses are nice. Fairly cheap, and plentiful. Check out the style and quality of the GMC-RTS. THis was the choice I made for my first conversion which I am working on now. Good sources tell me the RTS is the easiest bus to convert to a good looking coach, and I am convinced of that. They are very well designed and built like a brick youknowwhat! Drawbacks... Slow due to low gearing and drafty because of split door design. Also, no under floor storage like highway coaches have, however get creative and you can design and build some limited storage areas. Highway coaches are more expensive, but they are at home on the highway. Complex systems, maintenance issues, but the resulting coach can be a masterpiece. (Be prepared to throw down some $$$) By the way...another thing I really like about my RTS is that most of the frame is stainless steel, and skins are easily replacable fiberglass panels...if you get some other bus with steel frame members check for rust. You don't want your bus to break in half if at all possible...which brings up another suggestion...put heavy thingd above or near axles and wheels...too much weight in the center of the bus can really mess things up, I am told.