If you want an 850, my Volvo mechanic(dealer) would recommend a 1996 model year. He claims to see less problems with that year and it is in your price range. I fyou can determine the Evaporator has been done recently then that is a good start. Be sure to run the A/C long and hard on your test drive, sometimes they run for a while and then click off. That is another problem and not the evaporator. The car is a beautiful car and with some care should give you good service. Parts are in good supply at places like FCPGROTON. Like the other posts said, the problem areas are A/C, rear engine seal, ABS controller. Good luck in your search and I wish you well in school.
1996 850 GLT 225k

p.s. The Light Pressure Turbo in mine is awesome and has not been an issue.