Mercedes 190 2.5D. 276,000 miles 25 years old. £500 on ebay 4 years ago.
It's slow and handles like a cow on rollerskates so my licence stays clear.
Runs fine on veg oil and I get about 45 mpg - 50.5 best but that was driving like a vicar. Most comfortable and reliable car I've owned. Easy to service. Fantastic build quality. I can get 100 litres of used oil a week so I'll be making biodiesel soon.
Would kill for a Citroen DS though. - Early 70s one - not the modern type.
Shopping for a R1100GS as well.