If you dont know what to look for then best thing is to let the proffesionals do it. If you havent ever driven an R33 GTST and dont know what they feel like then just driving it isnt go to tell you anything also.
You could go into detail regarding accident history also. Checking for off coloured guards, orange peel, etc.
If he has documentation that states he has replaced the timing belt. Then in most cases it would be ok. But if he says it has been done and cant provide any evidence then get it checked or just replace it.
I could go into detail about checking everything, but I cant stress enough how important it is to get every little thing checked out by people who know what they are doing.