No, working on an A3 is very simple, you pull out the current unit, install a GPS antenna and maybe a few minor tweaks depending on fitment of Bose, BT etc and then just drop the new unit in. It will also need to be coded using VAGCom or similar, but the parts should be circa £500.
The problem with the A4 is that it is built into the dash as modular units rather than the 'headunit' design of the A3.