Hmm something like a modern TDI that will make the wife happy...
I'd think late W124, like a 1994-95 E300D. Or an early 90's 300D with the 2.5L 602. Both will deliver 30mpg+ on the highway and are very durable and tough car/engine combo.
If you want to spend more, a 1998-99 E300D is a sweet ride.
