You need to figure out want 3G options you want fitted, as there are 6 different versions of the main Audi Multimedia unit, radio module with and without DAB, you will need a new bose amp if bose if fitted, you will also need a new screen. Best place to buy parts would be eBay.
to upgrade it to mmi 3g you need:
Mmi 3g high resolution screen approx £70
New radio/AMP module with DAB. Audi no longer have a separate AMP for cars without Bose/B&O it's now part of the radio. Cost £160
Audi Multimedia this replaces the old control head/Nav/BT and AMI modules. The unit I picked up was a hard disk based version, but does not have the sim card slot. Cost of this was £170
Final module was a new CD-Changer with the 3G setup cost approx £80
Aerial Amplifiers DAB/TV
Digital TV module
AMI Cable
Multimedia to screen cable
V2 climate controls
You also need to get the can-gateway software updated, for MMI 3G you need software version 0071 or 0080.
hope I've helped.
Cheers
Steve