maybe look for if it pulls to one side, the steering wheel not being stright when the car is traveling straight. see if the core spport for the radiator is bent/damaged in any way.
The red flag to me is that after investing $6k in it over the last year or so, he cant fork over the $800 he claims will fix it all. must be something he knows that he is not telling.
Either way at that price it might not be a bad investment. good cheap fun, find another turbo car for under $3k...