I’ve been looking at exactly this, the A6 is only available with the 3.0 in quatto form, there are some deals about for the outgoing model, but it’s still pretty expensive. I think it should do mid/high 30’s for economy.
I had a look at the Octavia and the Superb yesterday, the Octavia is still quite a big car. The Superb is massive, looks like a lot of car for the money.
My Mrs has a Tiguan 140 w/four wheel drive, and it get’s mid 40’s (mpg). The four wheel drive is fantastic in the snow/wet. I really want my next car to have it, but it does limit your car choice a lot.