Cat D is usually quite a small problem... and like you said, a year old Audi S6 is likely to have been worth a fair wedge of cash so would be worth finding out why it was written off. Is there a place you can do this??
It could have been stolen and the Insurance Company had paid out on it before it was recovered??
Would have to be really cheap to tempt me though, cars like these which are HPi clear are pretty unloved at the moment.
Cheers,
Grant