If the offset and hub spacing is compatible with your 2003 they might fit, assuming 18" tires aren't so wide they would rub. I believe the hub spacing is 5x112. As I understand it, not all VWs use that hub spacing so you'd better make sure of that. Not sure about the stock offset, but you should probably be somewhere close to stock offset assuming all else is stock. I guess the short answer is you should tell us more about what wheels you're considering and then we can better assist you.
I have 18's on my 2003 GLX 4Mo Wagon. They are Factory option wheels for a 2016 GTI. They have 18-220-45 tires mounted on them. I got them from a VW Dealer after the guy that ordered them did not like them.Got a great deal. Improved the handling a great deal over the stock 16" wheels.