Hey
If you are on a budget as far as suspension goes u can get a good kit from G-max , it cost about ?250 for the "sports kit" (dampers and springs) and gave me a drop of around 60mm (if you are going low, watch out for the arches, but i think mk3 have a bit more room??)
As far as backboxes go, hmm im having to think about refined sound, as the only ones I can think of emit a deep burble on tickover and a throaty howl when in motion
Well im sure other people will chuck some more ideas your way, good luck with it all
Cheers
Ows