When I was in Highschool I had an 03 gt and I worked more hours than I spent in school. Modding will get expensive quick, and once you start, there's no end.
You want something cheap to make it look good, look for used parts.
If your car doesn't have bullitt wheels, find a set of bullitts for cheap and paint them gloss black, if you take your time they'll look perfect for ~ $10. Then sell your stock wheels.
Either get some used lowering springs, or cut your stock ones.
You can get an o/r x pipe on ebay for $130 shipped.
Get a rear blackout panel decal, or paint it on yourself.
Black out your tailights yourself, or get some film tint on ebay for $10.
Paint your hood scoop flat black and get a flat black hood stripe.
You can do all this and barely put a dent in that $600. If you take your time and do it yourself, it will look like you've spent a good amount of change on it. Ask me how i know