The OM603 is a good engine, but if there is any cooling system failure && the driver keeps going FORGET the engine!
The OM617 engine has a cast iron head. Somewhat more forgiving, but they can fail also.
The W140 chassis diesel weighed too much for the size of the engine. Oil consumption is common & other(non-diesel) failures are somewhat common.
The 1998 E300DT is a OM606 engine.