Hi mate,
I think it depends what your plans for the car are, if you plan on doing a lot of modding an STI is gonna give you a better start, especially if you get one with a PPP kit as you are already at 300bhp and get the bigger brakes and 6-speed box like you said. Things to watch are knocking shocks as the standard KYB untis are terrible and WILL fail at some point so make sure if you hear any noises get them to stick a new set on. The engine is pretty much bullet proof in 2.0 form but i know the 2.5 from word of mouth is not as reliable. Head gasket and piston ring problems so be sure to make sure its been well maintained. If you search on here for threads about the 2.5Ltr engine you will find out all about it. Im not saying dont buy 2.5ltr as im looking at one in the near future i think you just beed to be more cautious and i have been told they cant handle as much power reliably as the 2.0ltr.
In terms of running costs i went from a 1.5ltr tubro mitsi colt averaging about 35mpg to a 2003 STI and the loss in fuel economy does take a bit to get used to. On long runs high 20's (I managed 32 on a 150 mile cruise at 75/80) are possible but average of 23-25 for normal driving. Pushing on and your 15 and falling, lol. Things like brakes and pads are not much different to most performance cars. Its all about what your willing to spend. Good performance friction kits can cost upwards of £500 for the best quality brakes and if like me you dont require serious stopping power then basic standard discs are about £120 pads around 50-80 depending where you buy. Again you get what you pay for. Ive had mine for a year and its a fantastic car, its not missed a beat and has never let me down. In terms of wear and tear in the year ive had it its needed a wheel bearing, a drop link a new battery and it had a new radiator but these are things that could affect any car. Subarus are generally good for reliability and my engine and gearbox are solid and the cars done 78k miles.
Hope this helps a bit and welcome.