Painting the plastics is troublesome as they're textured to need to be smoothed first. A lot of work involved in that, so unless you're doing it yourself it is expensive in labour costs.
As far as chin spoilers go, everyone and their uncle goes for the VR6 (with good reason, they look great) but there are other options...
http://www.golfgtiforum.co.uk/index.php?topic=178674.msg1678701#msg1678701Venom motorsport do good prices on exhausts. I think a cat back miltek system is under £300.
Changing the grill for a debadged one, or painting the badge is another option. Or a Vento front end? Can all be got as genuine VW stuff. It's a big mod, that's stunning and isn't in your face.
Google Lamin-X, they do tints for lights and such, good simple mod.
debadge the back and de-wiper the tailgate, pressed plates, smoked or clear foglights... All can be got on ebay...