Re: Potential new owner –
Yes the 3.0Tdi is worth it - this has been discussed extensively so do some searches - but in summary the A6 is a big car and the 3.0 really suits it. Many people with the 204 and 245BHP units report similar fuel consumption to the old 177BHP multitronics. Certainly it seems easier to achieve closer to the factory stated MPG on the bigger engines.
Unless you're going to hunt down a manual gearbox, at the age you are looking at - pre-facelift - the 2.0 is the multitronic gearbox... Some people love 'em some don't. But they do require servicing.
The 3.0 Tdi 204 front wheel drive was also the multi box... For the 3.0 Tdi 204 engine the "exception" is the allroad - they are quattro and s-Tronic.
The 245BHP 3.0Tdi is quattro and s-Tronic. The 313BHP BiTdi is conventional auto box (Tiptronic ZF unit).
Standard spec at the age you are looking at is 2-zone climate control - you have 2 dials in the dashboard that you can use to adjust zircon temp etc. on the left or right side of the car. If there are adjusters in the rear it has been upgraded to 4 zone climate. You also get the digital temp readouts in the front as part of that and (apparently) the aircon is a bit more effective. Some folks with 2 zone complained that it was not as effective as the 2 zone they had in their C6 generation cars. The 4 zone in my car is at least as good as what I had in my C6.
If you are only doing 8 or 9k a year then I would say go for the fixed service interval rather than variable as you probably do quite a few short trips. It will be around every 9.5k miles - so once a year or so. The variable is unlikely to stretch to anywhere near 18k unless your 8 - 9k miles consists of lots of long trips and I would not want to leave old oil in the car for extended periods.
A6 allroad BiTdi (MY14, July 2013 - DAB & power tailgate). Glacier white, tech pack (HDD Nav, colour DIS, Parking Plus), sports seats, 4 zone climate, heated seats front and rear, ambient lighting, 75L fuel tank, adaptive Bi Xenon headlights