Save your money, not trying to bring you down but the only reason id put money into your car is for maintenance or if you found a smoking deal on an STi/WRX swap which in turn doesn't make it you daily anymore. Although it is fun today mod a car, Id get the thing 100% up to date maintanence wise, then save some cash for future maintanence, then start slow with bolt ons.
Myself, being older and wiser now, wished someone could of tell me that advice at 18 lol. I now have gobs of money into performances parts, and now I have to do a clutch/Fly, Timing Belt, and most likly wheel bearings. All of this while getting a 1200 plus tattoo, then xmas and GF's birthday. It just sucks cause it usually all happens at once too. I have the money for all of this now, but thats my house fund lol. Car parts < House Fund