Basically, before you even pay for the car, check out the pictures properly and look for bits of rust around the body. The japanese agent i use always examine every bit of the car very cautiously. That's a real big plus point of using him.
Most likely, you'll look at changing a new clutch. Certain clutches will not hold the power of the car. Normally GTR gearboxes are pretty tough, so that might not be a big worry. Don't forget to do a full flush of the brake fluid, oils, etc.