Ketilh,
I'd be interested to hear how you get on upgrading... I would like to do the same... I am about to take delivery of my new A6 and I also could not bring myself to spend SO much money on sat nav...
I would also be interested in upgrading the MMI Plus system in the car to the TV and maybe DAB too... the only concern is upgrading the MMI is one thing (apparently it's like plugging in PCI cards to your PC.. easy! however for sat nav you will need to mount a reasonable GPS arial somewhere....
Check out KUFATEC...
http://shop.kufatec.de/index.php/cPath/2_111
THese guys seem to know their stuff.
Good luck
All the best
BSK