I own an auto SL myself, basically everything that has been said is what you should follow. While I'm all for some mods, there's no point in putting a
ton of money into it. If you plan to drop a lot into it, you could just wait to get a VR4, 100+ horses, AWD, AWS, and lots of other goodies. The earlier models are getting pretty darn cheap these days...
Now on to the part you actually care about, I've always used this guide
N/A UPGRADE BIBLE. by YOGURT!. It gives pretty much all the mods you can do to a N/A, as well as tell you about what they'll put to the ground. Any information such as where to by, or how to install can be found on the forums, or just post up and we'll help out the best we can. Hope it helps you out.
Melbowski