Oh gosh, this question has been asked a 1000 times here...generally , ot's just like buying any car as what to look for. Condition of the body,suspension, engine interior, tires,brakes,etc. A test drive will reveal much.
As for upgrades, the first thing you need to do is make sure that the car is in good condition before making any upgrades as these proceedures will put more demands on the car(engine,brakes, suspension,trans). Then I think the best bang for your buck upgrade is a boost controller to tap into the 6 or 7 psi available that's not being utilized. A very noticeable increase in HP.
MR2's are great cars with a lot of potential. you're gonna love it!
Check this out the sticky from this forum. Lots of good basic info.. here's the link, but it is in the sticky in this forum
http://www.mrcontrols.com/index.html