Welcome to the madness!
With 250K miles on it I would assume that many parts have been refreshed already. Just the same, have your mechanic look at brakes, U-joints, Rear axle bearings, front steering gear, and transmission as well as the usual engine bolt on's (alternator, AC compressor, steering pump etc.)
Unless you got service records with your bus you should have all filters and fluids replaced (oil, trans fluid, coolant and rear end lube).
Having an oil analysis done will tell you a lot about what shape the engine is in. Check out Blackstone Labs for the oil analysis.
My 2000 Ford has a magnetic oil drain plug and a magnet inside the trans pan. Metal shavings on either one can be a warning sign.
Look at the date codes stamped on each tire to find out how old they are. Unless the bus was parked inside out of the sun they should be changed out at 5 or 6 years old. Sidewall cracks are a sign that you'll be making a trip to the tire store soon. The codes are in a week/year format. A code of 2312 would tell you the tire was made in the 23rd week of 2012.
Your AC compressor is likely engine mounted with the condenser coil down on the driver's side of the bus and the cooling coil up on the ceiling in the back of the bus. If you won't be moving any of those pieces fixing the AC will be as easy as fixing any leaks and recharging the system.