I have a 1983 300D with just under 250000 miles on it, and aside from oil (Mobil 1 Delvac synthetic), filters and fuel additives, I have replaced 2 shocks (Bilsteins = expensive) replaced one ball joint, a battery, a used heater motor and two wiper blades since last spring (2001) Total: about $550.00 CDN.
Not too bad, I think. The Buick I drove before this car was a lot needier and less reliable.

My car does have one needed repair at the moment - it needs a wiper pump, which I have yet to replace, (cost as yet unestimated, will probably buy used & do it myself).
Oh yeah, and it will need tires soon.
Dont worry, your diesel will almost certainly go down in memory as your most reliable and inexpensive car. Maybe not as reliable as a new Honda, but what would a Honda cost to operate at 250000 miles? (and what would it look like?)
My quarter million mile MBz, and many others I have seen, are still absolutely beautiful cars. It is the best car I have ever owned, stacking it up against Toyotas, Volvos, a new Mercury Sable, and others; it is the best value miles-per-dollar that I have ever owned, out of the 47 cars that I have owned since 1972