#1 avoid rust
#2 avoid rust
#3 avoid rust
check everywhere for rust, including underneath. A bit of surface rust, maybe OK. Any more can likely mean serious problems where you can not see it. Avoid vehicles in the N.E. or anywhere where they put salt on the roads in wintertime.
Make sure the tires are newish. Find out how to read the DOT codes on the tires to see how old they are.
Make sure you don't see any fluid leaks. If the bus has hydraulic brakes, make sure to check the "back" (inside) side of the rear wheels and if you see any sign of moisture, it is likely a brake fluid leak, but it could also be an axle seal leak.
Knowing what bus you are looking at can help give better advice.
Full size or van-cutout, make, model, motor, tranny type, etc....