Do a search. Here's a link but it'll only get you started, you'll need to search the forum for more tips:
im gettin a new wrx...any suggestions?
Here's a quote but again, it's just a little info, search for a more complete picture of how to buy a used 02:
"Be meticulous and cautious. Do not buy a modified car.
Only buy a car with FSH in the form of a logbook and original receipts in the owner's name from an authorized dealer or repair facility (no independents for the FSH on such a new car).
Take it to an independent specialist and have them do a compression test, check the turbo shaft for play, check the brakes and clutch for wear, and pull 100 ml samples of motor oil, gearbox oil, and coolant - have these analyzed by an independent lab. Ask the owner what brand lubrication/cooling products are in use - if you get a blank stare or BS, do not buy the car. If you get an answer, send the samples in for testing and use your own fresh fluid (same type indicated by owner) to replenish the car if needed.
Have the mechanic comment on the plugs while they are out.
Have the shop pull any ECU codes. If no codes, check the battery terminal for wear indicating it's had contact with a wrench - if the FSH doesn't indicate any reason to pull off the terminal, the ECU codes may have been blanked by the owner. Ask and depending on the answer, decide...
Find a body shop and pay a body repair person to take a look at the body and tell you if it's been re-sprayed and if all the seams are original, and if any panels have been replaced.
Check the condition of the driver's floor mat. It should not look ancient in a car with low mileage. The usual pedal cover check doesn't apply, as these are metal. Check the wheels/rubber for ridiculous wear that doesn't correspond to the driven-by-granny-to-church-only story you may have heard.
Do not buy a car with a spotless engine bay. Find a nice dirty one. Look for streaks of fluid and/or other leak signs. Look for juice marks on the tarmac where the car is usually parked. Check the exhaust component bolts from the manifold all the way to the muffler, turbo and up/down pipe included of course, and see if they've ever been removed. Check the story.
Look all around the car and inside the engine bay for anything that looks out of place or that doesn't agree with the image a machine put together at a factory by pros and robots would portray - signs of an aspiring "tuner" should make you run.
This will take time and effort, and will not be free, but it will ensure you'll have a fighting chance of getting what you think you're getting. Don't rush to buy a car, it's simply not worth it. This is a Subaru, they make them every day and you'll find another soon enough. Rushing costs money."