From what I've seen:
Make sure the gearbox has been serviced at the correct interval, 40k miles. Make sure it drives smooth, no jerkiness. (if it's an S-Tronic gearbox). You might be able to get a BiTDI for that money which has a proper auto box.
Listen out for a stretched timing chain or broken tensioners, especially pre facelift.
I think they're the major ones for the A7.