I will second the idea to get one from a school district, especially if you can get a year or two of its maintenance history and can do a test drive around the lot. Ask the mechanic which is the best of the litter and you will probably get a correct answer.
If you are going to bid on one make your first bid in the last minute of the auction, any sooner and you are only increasing the price unnecessarily.
Do research at your states dept of motor vehicles on getting it registered and at your states tax assessor office on how to get it re-titled away from school bus to an RV, do this before you purchase. The govt employees will probably not give you correct answers because skoolies are rare so you must find the regulations yourself.
In texas I talked to several employees a supervisor and a manager of the local office and they did not think it was possible to re-title a school bus in texas, they had to call the main office in austin and one employee said it was possible but did not know how, had to find the regs myself and when I went back to get it done I had to give the employees the section number in the regs so they could read it themselves.