Barring anything major like collision or other major damage/repair, it's awfully hard to go wrong with a Jetta TDI. Yes, that's a turbo-diesel. A manual transmission is much better in those for fuel economy, but it won't do poorly on the slushbox either. Economy rivaling the Prius, with much better performance.
I'm a huge fan of diesels and think that the lack of a diesel engine option is where the hybrids are very seriously missing the mark. A hybridized diesel would easily get around 50% greater fuel economy than the current gas ones being offered.
There's a reason all modern locomotives are simply huge electric motors "fueled" by huge diesel generators. The engine you hear on the train isn't moving the train. It's powering the generators that power the electric motors to move the train.
If the car is in good condition and its background checks out, you should snap it up.
Keep in mind, though, that a downside of diesel is that it doesn't evaporate like gasoline, so adding fuel isn't exactly something you want the better half doing while wearing white gloves. My wife is averse to having a diesel car for this very reason. She doesn't want to deal with the mess that's always present at diesel pumps.
Also, diesel (at least in this part of the country) is currently costing a LOT more than gasoline. It used to be and should be the other way around. Diesel gets very little refinement compared to gasoline. It's only a few steps removed from what comes out of the ground.
