I won't even try .. no comparison at all to the A6 Allroad .. but I have been impressed with it ever since I purchased it .... fun to drive, good handling, reliable, and when you want it to .. it goes !!! ( 2 speeding tickets in France attests to that !!) .. and extremely reliable
If you want it, and can justify it to yourself ... go for it 
I really respect your opinions on most things, and I have read with interest your views of ownership of the A6. (more below)
Guessing timing belt as 2007, not timing chain?
Either way, expensive and probably due changing.
Have you got option for a warranty for major components?
To be honest, I've been eyeing this car up for a couple of weeks. I've not even put a call in to the garage to ask them any questions.... but a MOT history check shows not a single advisory (except a couple of borderline tyres) in it's life.
I'm pretty sure all the 5.2's in the S6 & S8 were chain driven...... and at 116k miles, it would benefit from a change. It's something I've factored into the first year cost of ownership.
My concern is with the general up-keep.....
I've never had a car with carbon discs, I've never owned a V10

In general, do the brakes last as long/longer than standard steel discs?
Normal service items, like Oil, Coolant, Plugs etc wont phase me, as I (as many of us) find bargains and pick them up as-and-when rather than on the day of a service.
Being a very heavy car, are there huge bills when it comes to suspension parts etc. (I seem to recall the A4 had a whacky front strut arrangement).
Decisions, decisions.