I remember the first s-class MB I ever rode in - 1984 500 SEL gray-market car. It had 90k on it and was absolutely perfect. But, he said the only thing he ever had to fix was the water pump!

In my experience, the overriding factor for any water pump failure is the pulley/belt. Any small wobble in the pulley, bump in the belt, jerking in the tensioner can create excessive wear on the seal and lead to early failure.
That being said, pumps that don't have the fan blade attached to the end of the shaft "should" last longer. Belt tensioners that have little shock absorbers with "instantly hardening gromments" on the ends that rattle and bang are in for trouble (m103/104, etc). I've seen these start to bang and thump at 10k miles.
And, yes, they make some really bad aftermarket pumps.
I seem to get about 50k from my M103 pumps - Lasso seems the best I have tried.
Best advice; get a Lasso or other good brand, be sure the belt runs very smooth (just install a new one), be sure the belt tensioner and idler pulleys run smooth and the tensioner shock is good, adjust tension properly, use MB coolant, inspect for leaks/noise often.

DG