Hi Gareth (@Magnet), thanks for your reply, and for raising some questions to think about!
Like most A6 Avants, it sounds like the high mileage was down to company miles, and I would expect the early MOT was down to the dealer wanting to offer a full years MOT rather than a few months - one owner, but bought by a dealer who's selling the car now.
The wrap isn't brand new, as there are a few stone chips and scuffs, and the dealer said it was the previous owner who had it wrapped, so there's no reason for me to believe it's the dealer that's installed it. My thoughts are maybe the previous owner had it as a company car, then purchased it from the company, and had it wrapped at some point...possibly. I've spoken to Audi (or at least one dealer who was helpful, and one who was less than...), and based on the service records there's no notes to say there was any repairs / damage etc, although this doesn't mean that there hasn't been any that they're not aware of.
The car is immaculate inside, which does make me think that the owner did care about it, so I would be shocked if they had spent all the money on a wrap to hide any paint damage - especially as the paint damage would show through.
As with any wrapped car, I think it's a risk, but I've got to decide if it's a risk I'm willing to take, based on the lower price compared to others.
Thanks for your advice Gareth!