There are plenty of used sti's out there, the key if finding out how the owner has treated the car. If he's out beating the piss out of it, your going to have issue's. I would also check with a local Subaru or the main office with the vin to see if the car has been red flagged. Just beacuse the car is Stock in NO way means it has not been modded before you had the privelage of looking at it.

Example, there is a 04 STi in the for sale section with 4k on it for peanuts. The insurance wont be that drastic from a WRX to an STi, and once you get the WRX you will wish you had the STi.