The Brembo 18Z of the Touareg/Q7/Cayenne cost around that value (£350) and the adapter bolts are cheap. Then you need centering rings for the 350 mm ML450 discs. You can get the discs and pads relatively cheap as well. You can either get braided lines for Brembo calipers or for GTI, they both fit well. Just another alternative for a good BBK.
The TTRS/RS3 calipers are expensive because of the 370 mm TTRS discs. If you get the adaptor bracket for R32/TTS discs you need to pay £260 (includes braided lines), but then you can use cheaper and lighter discs.