As far as WRX's go the biggest thing i would be concerned about is the transmission. The "weak" spot is the shift from 1st to 2nd. If the person has been slamming gears or powershifting it will definately mess up the tranny. Right before my transmission fucked up, second gear felt very "sticky". It was rough to get into gear, and it would click or grind if it was put into gear without a little bit of finess. The tranny was also making alot of noise under deceleration w/o braking.

Another good thing to look at is to check the spark plugs. It you see white residue on the spark plugs thats usually a good sign of detonation within the motor. cylinder #3 is usually the "weak spot" on the EJ20 so i would definatley check that plug for signs of detonation or burning oil.

Even as far as somethign simple like oil changes, check the area where you actually fill the vehicle with oil. If it's nice and clean under the cap i would be a little more skepetical about proper oil changes(unless they can produce proper paperwork) than if there is some oil residue built up under and around the cap.

I'd check around in the engine bay if it's a "stock" car. I'm not trying to stereotype anyone, but i wouldn't be too anxious if your buying a WRX wagon from an older couple who say it's stock than buying a WRX from a 22 year old in the military who says it's "stock" and wants to sell so he can get the new EVO. Check some of the things like the screws holding on the stock BOV to the intercooler. They are VERY soft and use an allen wrench, so if someone has removed it or fucked with it you will see the screw has been rounded off a little bit or it will have a few scratches and chips in it. If they notice this and say "oh well..it did have a BOV but i put it back to stock" that would make me question what else was "put back to stock". The same could be said for the bolts connecting the exhaust. Those things can be a big pain in the ass to break lose, so look to see if they have any rounded edges or scratches on them.