welcome shaun,
well the subaru is a ideal car for the events your thinking off, Engines are cheap enough now and can be changed in a day if your fast with tools and got the knowlegde
You would prob be best off looking for a crappy shell classic uk turbo i would say, near as standard as possible! wagons are a bit cheaper (same floor plan etc just a bit bigger boot vertically) plus they are better in every way possible (not biased)
Engines:
Well the uk is known for being a weaker engine, I have ran em at 340-350bhp which is what the limit is really on the gearbox, the gearbox in your case might be the best upgrade! you can change the box and driveshafts to a stronger sti unit for a few hundred quid.
BUT
if you got a cheaper car there are loads of nice spec cars being broke, for instance you can pick up a running 2.1 stroker for about 12-1500 quid! or a decent forged sti block for under a grand with a closed deck block!
After market parts workd is endless, Anti roll bars and suspension need to be upgraded tho! whiteline, cusco, bilstein do a lot of products and some by CDF racing are ok too!