I recently ran a ZTT diesel 54 plate it was a nice car had to put new clutch in £1k plus new alternator,not as economical as you would expect 35 in normal use rising on longer runs.The motors have one or two inherant problems you must keep on top of ie keeping the ecu dry .

Certain parts are like hens teeth to get but normal service parts are available and reasonable priced.I would think that the main problem is finding a car in good condition most have mega mileage especially diesels.To widen your choice look at Rover 75 tourer identical car don't forget the last one was made in 2005 ,I would look on the various specific forums and see what the common faults are overall a very nice well put together motor unfortuneatly obselete.

There's a large element of risk in buying any used diesel, as even with a basically good car like the 75/ZT one expensive repair negates any fuel saving you were going to make.

I suggest you either keep the one you've got till it starts needeing uneconomic repairs, or else - oif your annual mileage is high enough to justify it - think about an Octavia diesel. The 1.9 TDI seems to have the best reputation for reliabiltiy and longevity, and of course you will also find this engine in other VAG models. You'll get a newer Skoda for your money than a similarly-powered VW or Audi.