About to Buy a Used Class C, Looking for Advice

YOU can check the date on the tires, and double check the cieling for water spots ,and the corners IF you see water marks RUN ,does the frig work?? and $500, to fix the air sounds cheap, I,d have some one else check it out, this is no time to be shy ,rv,repairs are big bucks, maybe spending a little more on a better unit is cheaper in the long run.