I haver a 6.6L duramax in my Chev. Its a fuel pig. The newer North American trucks mileages are digusting. Most of that doesn't apply to a Euro buggy however.
I would look very closely at how much oil changes are for the model of car you are looking at and factor that into your costs. My truck costs over $100 for an oil change. 50K would be about 10 changes per year. -- Some hero's wear capes, mine wear combat boots.
I'd probably be looking at a used Jetta or Passat TDI wagon. I haven't started down the used car pricing road yet, so I might be unrealistic about what I can expect to pay. It's not a crisis yet with the current car, but it could come to a head soon if things start breaking down. I don't like some of the noises I'm hearing from under the hood these days, and the consumption has really increased in the last few weeks.
My Jeep is diesel and I get better fuel consumption than the same model with any Gas engine (you can get a 3.7l and 5l Hemi for my year and model in gas).
Mine is a 3l Turbo diesel and anything that has went wring with it has been either electrical or just a common thing (glow plug).
My gas mileage is not great 12l/100km on average because the vehicle is in 4wd at all times (no 2wd option otherwise I would save more on gas). -- "The hardest thing about any political campaign is how to win without proving that you are unworthy of winning." ~ Adlai E. Stevenson