what's up man? I'm 15 too. Got the car when I was like 12 or 13 though. It stayed at my grandparents house til I was 14. Yeah the stock Si hp was 108 or 109 depends on who you ask. The STD(the one I have) had like 78 hp and no tach. This was due to diff. cams, headers, and fuel injection systems, and some other things I can't think of.
Ebay isn't bad as long as you trust who you're buying from.
400 is a pretty good deal on the wheels, I got mine for $500 with toyos but they're only 15's.
Hmmm... cheap places to get parts... what kind of parts do you want?
www.nopi.com's E-cat is always a good place to start, and Ebay's good too.
The turn problem sounds like an axle but don't take my word for it, it's hard to narrow something down from just what it sounds like, it could just be the cb joints. Get someone to check it out.
they make vinyl paint for the interior, just clean off the surface really good and sand it really really lightly and it should stick.
I think that answers mostly everything, or makes a good guess at least. Good luck with the steerin problem, it shouldn't be too expensive.