Being a 2002 it has what is considered the weakest transmission out of the model years. You want to make sure that it has the stock clutch (or atleast not one stronger then stock).
I'd also look for a vehicle that has all it's maitance done on it up to date. There is no reason why someone shouldnt be able to show you proof of oil changes/maitance.
Also with the car being a 2002 it will probaly have a fair amount of miles on it. Oil pumps in these cars tend to go around 90,000+ miles so if you are past that make sure it was done. These are typically done with the timing belt, which should also include a water pump if the person had some common sense.
Other then that make sure the body is straight and it hasnt been abused. These cars are not "rare" by any means so pass on something that is heavily modded or looks abused.
If you do not know anything about modded wrx's then I highly suggest staying away from them. Buying someone elses headache of mix matched parts can be a PITA.