If you only have $500 to spend on music then I would recomend focusing on your Bass First

The 2000 newer VW's should have have a descent system with it already unless it has been seriously abused already.

So what that means is add some nice bass and $500 can definatly get you some descent bass for inside your car and then later when you get more money you can start upgrading the rest of the system a little at a time.