Buy an older STI for the same price or less? The problem is you're already locked into this Impreza, if you don't keep it 3 years, you'll lose your shirt. You're probably underwater on it 2-4 grand at this point (meaning it's worth less than you owe on it). If you engine swap it, it'll be worth even less. Hell, just kiss the warranty goodbye and any resale too.
I think a non-turbo impreza could be fun to mod. Wheels/Tires/Brakes and suspension. All the good stuff will fit.