Glad we could help Phil :)
You are being wise not to rush a big decision like this.
As long as you do a motorway run once a week then you'll be fine and not have to worry about the dpf. The dpf clogs with soot because the engine and exhaust doesn't get hot enough to burn it off the filter. For example I do 500 miles per week and have a diesel and have no issues. My driving is a mixture of motorway, country roads and town driving but the large mixture doesn't present an issue.
However, if you did 20 miles to work and back then this would not be enough to keep the dpf clear as the engine won't get hot enough.
If the dpf had to be replaced you'd be looking around £1000-£1500 approx to replace. These figures are estimates from what I dealt with whilst working in a volkswagen service department.
The 2 wheel drive versions are more than suitable as most of the time Quattro is not required in our climate. Unless you live in certain parts of Scotland or Wales. The beauty of the a7 is that despite most of them being base spec in front wheel drive the first owner may have chosen nice options as you've found out.
In terms of spending large sums on your a6, I can fully appreciate where you're coming from and to be fair if I wasn't sentimentally attached to mine then I'd probs have got rid of it. Running a high mileage a6 can become expensive as I know from experience.
Cheers
Steve