You need the TPMS control unit, 4x receivers that are fitted in the wheel wells, then 4x sensors in the alloys. The parts will set you back around £300-£400 + you would need to make the wiring harness up, and you would need a tyre fitter to fit the new TMPS valves & have the wheels balanced again. You would then need VCDS to code the, can-gateway, MMI & instrument cluster. It's not a quick or easy retrofit. Also make sure all the the sensors & receives are 433MHz, as Audi also sell 322Mhz but that is for the US market