If your going down the 3G+ route then it not a straight forward retrofit like on the 3G retrofit. I've now replace the 3G setup in my existing post and fitted 3GP, it requires a 5F unit from either a Q7, Q5, A4 or A5. You could convert an A8 unit but that requires changing the MU version, as you need MU9498 with software version 767, (MU9408, will not work with the A6, C6 as it uses different MMI software, also A1 & A3 units will not work). You also need to figure out if you want to have the 5F module with or without SIM slot.
I've not found a single site that has a full break down of a 3GP installation, I had to trawl through German and Russian forum sites with the help of google translate, and there a very few English forums where people had done this on a C6 and if you do have question not may people what to give you a response, it seams very cloak to dagger as there are a lot of people try to charge for this retrofit.
I'm going to put a post up on my 3G+ retrofit, but it won't go into the detail making up wiring harness, you need to get yourself a copy of elsawin, and the wiring harness pin's as if you buy the repair pins from Audi it will cost you a small fortune just for the pins.
Also you need to fit a Q7 can gateway as non of the A6 gateways are compatible with the 3G+ system, and you will need to replace the central electric module with a facelift version, replace the alternator voltage regulator with a A6 facelift version and remove the old battery monitor as that is now done on the the can-gateway rather than a separate module like on all pre-facelift A6's, it also requires a new battery earth cable and there are a couple of pins that you need to remove from the old battery monitor to facelift earth cable....
When I get chance I'm going to post up a mini write-up of my install but 3GP is not straight forward, unlike the old 3G setup.