Have you studied up on WRXs? There's a lot of great newbie information available on North American Subaru Impreza Owners Club. Basically, on the stock turbo, the FMIC, BOV, and the boost controller are a bad idea. The WRX 5 spd. is weak, so if you treat it like a 240, it's going to take a crap. Have the car inspected by a Subaru mechanic, then, if you get it, look into turboback exhaust, up pipe, intake (some don't mess with their intake), and a tune. That will bring you to stage II, which is decently quick on the TD04 WRX turbo. After that, look into a modified WRX transmission with PPG gears or an STi 6 spd (make sure to go with a good clutch), and go for a turbo like the VF39 or 18g. Then you can look into a FMIC (more for the latter turbo). BOVs typically require tuning unless you want your car to lose performance, turbo timer is a good idea, and coilovers and wheels are a must.

High mileage isn't that big of a deal so long as it checks out; my car is about to hit 70 and she looks and acts 20!