I had a 2005 A6 SE 3.0TDI Quattro prior to having the zed, purchased second hand in 2009 for around the 15k mark, was traded in against the Z in April of last year. Was the saloon version, great car but obviously totally different kettle of fish from the Z. Effortless motorway crunching and an extremely nice place to sit, very comfortable also. Plenty of space for 5 adults to sit comfortably and enough space in the boot for another two, maybe three in the case of the Avant (only tried once), but used to be able to get all our band gear in the boot when travelling to gigs etc
Only problem I encountered in just less than 3 years of ownership was both front wheel bearings but apart from that not a single other issue, and maintained on 20,000 mile service interval.
I really liked my A6 and if I'm ever in the position to require a large saloon again the A6 will be top of my list.