I am in the process of buying an 09 Sepang RS6 Avant and have just been through the same thought process.
What I did is look at all available options/colour combos etc and come up with the car that I would have bought from new. Then set about finding it.
Top of the list, as mentioned, should be FASH. It needs to be looked after as I would have from new.
I then decided that Sepang Avant is what I wanted, with black leather. Options required were Bose, Privacy Glass, Heated F/R seats, Advanced Key, Soft close doors, power boot, MMI High with TV, sun roof and Homelink. A 700+ PS tune and Milltek would be added bonus
Took me 3 months but found the car, had to compromise on Homelink, but the price was good, history good and everything else was there. It even has the soot-pipe and an MTM 702 BHP tune, but this may be changed to an MRC stage 2 + gearbox map reasonably soon. A very helpful forum member checked it over for me as I was 3500 miles away; it was great to have this check before I flew in to buy it!
The only issue was the 'dreaded oil weep' but as I have time before collecting the garage is getting this fixed. And not with an oil additive
full engine out job. The showroom have been a pleasure to deal with, bending over backwards to accommodate me, that's an added bonus too.
Really looking forward to collecting the car when I fly in for Christmas, will be interesting to compare ownership to the XFR I have just come out of. One thing I hope MRC will be able to change is the gearbox reverting back to automatic when in Sport. I don't like that when you use the paddles in sport it decides to drop back into automatic after a period of time; nothing worse than eating up B roads in 4th and, just as you come to a long sweeper it decides to drop into 6th
The XFR would stay in paddle mode until you told it not to.
