If you buy GTI/GT170 carriers then you can keep your calipers, dust shields and hoses. Effectively going from 288mm discs to 312mm with same pads - I noticed on my TDI140 it was better at biting down, and will be the cheapest upgrade. Minimum OEM wheel that'll then fit is 16".
Next up, but staying OEM is a 345mm R32/S3/Cupra setup - great setup but fairly expensive. You'll need the whole setup and that includes hoses, dust shields and a brake bleed. Minimum OEM wheel that'll then fit is 17".
I may have a set up for sale in a week or so, depending on whether my current ones fit my mates Audi TDI that he's buying.