If you're looking for a vehicle you can cherrish and love, I would definately consider the body before all else. If it runs but the body doesen't have too much life left in it, then you'll be dumping in tons of money restoring it.
If the body's good and it needs mechanical work, it's easier to bite off a bit at a time.
Well, that's my opinion anyways.
Definately check out underneath the car, particularly where the suspension components connect to the frame. Also look for evidence of bondo work. If there's a lot of bondo, it could be hiding a lot of rot.
Anyways, good luck, busses are real cool, I hope to own one someday as well.
Garry