If you are looking for a reliable car get the 2.8 or maybe even a 3.0, but honestly my a6 has had a few problems. The thing is I bought my 2.7 at about 50k miles which is where a lot of the faulty stuff goes wrong. If you buy a car with about 80k miles most of the common problems will have already been fixed. Thats my take. I love mine and I also love my CEL's that come on and off daily. Big tip is to buy the VAG-COM or go to autozone for free to pull the codes. Chances are its nothing, but thats not what the dealer will say.

 

The older A6s are rated lower in reliablity than comparable luxury cars, esp Japanese. But recently VW has been near the top and both VW and Audi (same car company) have much better ratings. So a new A4 should serve you well. It's also smaller than the A6, which is nice if you rarely have rear seat passengers or luggage. But if GF becomes wife + kids, it's cramped.

I treat my A6 well and it has been reliable. I bought it with 37k and it now has 118k. Oil changed every 7-8k. It had brakes under warranty at 40k and again at 115k (+ rotors). I had an alternator fail unexpectedly at 75k. I did replace the secondary water pump at the same time I did the clutch.

Why clutch? I chipped the car and increased the hp from 250 to ~318. Then ran it WOT. I think the power at 150mph lunched the clutch. But...the previous owner had done some towing. So I replaced it with a stage 2 performance clutch and it has been fine. Total maintenance over 4 years probably $2000-2500 not including clutch/flywheel. Message - if it has been taken care of and you don't abuse it, it will be reliable. But keep in mind it has a lot of amenities and quattro adds a lot of drivetrain complexity - so more to go wrong.