My girlfriend had a 2005 sti, when she bought it the car only had a turbo back exhaust and a few little things. Drove it for about 4 months then decided a tune (ecutek)/fuel pump/cai would be added. Car went on dyno before the tune 161kw and 278nm after tune 199kw (270hp ish?) and 365nm.

The power difference was great, 270hp atw is plenty of power to play around with, you might as well enjoy the power of the car get use to it then decide on what you would like to do next.

If you have 4k make sure you at least spend 100-200 dollars to get the car fully inspected prior to your purchase, it might just save you heaps in the long run.