ITB's aint gonna be cheap man, but they'll be worth it just for the noise... nothing better than pure NA induction raw! I don't know if the standard ECU will be able to take a popper ITB map tho, if not, engine management aint cheap either! if you're thinking of goin down that line it'd probably be worth while getting the head done as well to maximise flow, i.e port polish, 3 angle valves, high lift cams.
It'd be interesting to see what can be achieved from an NA 20v, dont think it's really been done much. It'd follow suit with uniqueness of you're merlin purple 5 door as well!
No doubt there are a lot of people on this forum that know a lot more than me about this sort of thing tho... Either way, +1 for the AMG rims! I know they're not VAG but i think they look awesome and are still technically OEM, not too over-stated.
Good luck with the build and what ever you decide to do but as always, it's basically gonna come down to how much £ you wanna throw at it, however you have the potential and ideas for a completely unique car here! :thumbsup: