almost every mk3 turbo supra blew its head gasket around 200'000k
but your 85 is probly still going.
as far as air conditioning goes, its an old system and probly needs to be switched to the new gas type. it can be done, and people do it all the time. do some research over at
www.celicasupra.com you will find all the parts you need there and all the info. if you need any help finding parts let me know.