I don't know anything about retro-fitting the official Audi bluetooth kit, but I've just fitted my old Parrott CK3000 bluetooth kit into my new A3. They only cost £60 or so new (similar to the Nokia bluetooth car kit) and it works just great.
All it needs is a live, ignition and earth connection power connection from the fusebox, and you then connect the mute and speaker wire into the back of the radio unit - and the system cuts out the audio system and plays the phone through the car speakers, just like the integrated Audi system.
If you do want a cradle, you can fit one, or just leave the phone in your pocket...
I use a TomTom satnav unit on my PDA/phone and just use a simple and neat magnetic mount that attaches to the windscreen by a sucker.