Search for the "so you want to buy a used wrx" thread (can't recall exactly what it's called).
There are lots of tell tale signs about which cars to avoid like the plague; it's often easier, and far more thorough, to have the car inspected by a pro. If it passes all the basic inspection process then I'd pony up for a compression/leak down test, a couple of hundred dollar expense that can save you thousands.
Don't fall in love with a car right away and buy it without knowing everything about it. Things can get expensive if you end up buying someone else's headache.