I bought a silver 3 door MK4 R32 in 2013 with 50K miles on it with a view to convert it to DSG. It's been done in Europe but not in the UK, to my knowledge. The best gearbox matched to the best looking R32, what's not to like?
The only mods I felt it needed were genuine 'Ocean' rear tail lights, US seat covers for the Koenig seats (3 partitions instead of 2, which stops the common leather sagging problem) and a remap. Not to make masses of power, but to smooth out the horrible VW mapping. I did come away with 265hp though, which isn't bad from the 236 they make standard.
But after living with it for year I came to the conclusion it wasn't for me dynamically and the performance was also lacking. I need more than noise. I need the full package, so I sold it on. Kind of regret that now as it was a mint low miler, but never mind!
Another thing that bugged me was the exhaust drone from the standard exhaust, which was horrific, and even worse with aftermarket exhausts! I welded the flapper valve shut in the end and lined the boot with foam sheets, which helped a lot.
R32 dos and don'ts:
DO get a MK5 with DSG, or get a MK4 and DSG it if you're feeling brave!
Don't bother with exhausts and especially don't bother with downpipes unless you want to kill the bottom end torque.
Don't bother with cams. Total waste of money.
Don't bother with induction kits. Total waste of money.
DO Turbo it
The MK5 is the far better car dynamically and has the better engine, but VW put next to no effort into it's styling. If one comes towards you, you could easily mistake it for a Jetta. The MK4 was clearly handed over to the Motorsport division and it shows. H&R springs, H&R ARBs and Bilstein B6 dampers as standard
That is why the MK5 will fade into obscurity and the MK4 will appreciate. It was the first mass market hot hatch with a great big 6 cylinder engine and 4WD.