Whats the asking price on the car? In my experience, supras command more money than rx7's of the same generation, and the MkIII is hardly worth more than a rx7 (imo). N/a's of course are dogs, but I'll take a n/a rx7 over a n/a supra any day. Mileage is also an issue with older cars.
Many 7mgte's spin crank bearings if oil is low. Also electronics can go bad after years of heat and abuse, one noticeable sensor is the TPS, and an unstable idle. Oil leaks are also an issue, if you really want the car, just know exactly where the oil leaks are comming from, and negioate from there. One final note is the power steering high pressure line almost always goes bad as well.