I had a 1984 W123 300TD Turbodiesel I bought with 356k miles on the odometer (which was broken, total mileage unknown). I drove it for over 100k more miles before I sold it. It had no blowby and no perceptible oil consumption between changes. The OM617 with MW pump was a fantastic engine. The Garrett turbo had something to be desired though so I replaced it with a much more efficient Holset HX-30 which worked great with the pump maxed out. I estimated from 0-60 times it was putting out around 150HP, up from 120HP stock. The 722.3 transmission didn't give me too much trouble either but I did rebuild the valve body with a shift kit to make it shift better. The one major issue I ran into was the rear hydraulic self leveling suspension. The hydraulic struts were NLA so I pored over a bunch of parts manuals and eventually found a Lesjofors spring that was the right height and spring rate--I believe from a later model S600--which worked perfectly with Bilstein HDs from a W123 sedan. Should never have sold that car.

I currently own a W210 E300 Turbodiesel. I bought it with 49.5k miles, it currently has 120k. Overall it's been a decent car, the OM606/722.6 drivetrain is great. The rest of it is pretty miserable though. I would like someday to swap this drivetrain into a W124 wagon, with a standalone transmission controller and the injection pump from an OM603 to make the engine fully mechanical.

In the meantime, I'm working on rebuilding a 2.65 rear diff from an SL class car to swap in. I have a TCU from another car that had this final drive ratio so hopefully it'll work. The stock 3.07 ratio is no good for US highways. In 5th gear at 2250rpm (bsfc minimum) the speed is about 100kph (62mph). With the 2.65 rear it'll be more like 77mph which is where I usually set my cruise control. Should get a lot better fuel economy and less noise.