HI Jrinns,
Given you've been driving BMWs for a while (and presumably, that you like them), I'd hesitate on buying an Audi if I were you. They're a totally different drive to Beemers and it's down to taste as to which you prefer.
Take a look at this review of the Audi A6 3.0 BiTDI quattro in Autocar.
http://www.autocar.co.uk/car-review/...-bitdi-quattro
Apart form the usual comments about the Audi drive being bit sterile, they also suggest you need to spec it up a fair bit to get a halfway decent drive.
Given your space requirements, it might be worth test driving a 535d and then pushing your dealer on price against the Audi. If you can't get a sensible deal, you could look at a nearly new 535d.
Each to their own, but driving an Audi is the polar opposite of most BMWs. Worth pausing before you go ahead.