Nice job on getting a 2.5rs with so low miles! most WRX's dont even have that low miles! Anyways as far as the mods go its best to start out with intake, exhaust and ECU (i dont own one but i am going on WRX and other peoples experiance). Once you have done this, to really get it going you prob are gonna want to turbo it which makes the 2.5 one hell of a car. As far as where tp get it, I would say cobb tuning (www.cobbtuning.com) or there is a ton of other places.... just do a google search for "2.5RS tuning parts" and you will get a ton of crap. Hope this helps!
-Jeff