Hurdy if your going Quattro surely its has to be something with a longitudinal engine to get permanent drive and a 40:60 distribution, plus you can then get your hands on the 7 spd s-tronic which has many more clutch plates for smoother and faster pick-up. Plus it might be possible to swop it out for the R8 box or the Porsche PDK box? Which should take what-ever you can throw at it!
Or if you start with a R32 how about getting your hands on one of the new 2.0 TSI lumps with variable valve lift as standard they chuck out a considerable amount of extra torque, and I bet even a simple tune could release alot more without sacrificing the weight of a V6 lump.
What ever you do, will be an interesting build thread, proper AWD with decent front to rear power distribution 7 spd DSG, and a blinfding engine will make a truely awesome motor!