I spoke with the owner of Metric Motors (the largest MB engine rebuilders in So. Cal.) again today and mentioned this thread on the forum. He told me that there is no inherent problem related to valves, lifters, rocker arms or tappets in the W112 engine. He did say that he has seen problems with sludge build-ups which have caused problems in the valve guides, but that was only from a lack of routine oil changes.

He did not discount the possibility that some new engines could have been assembled wrong or had parts that were out of tolerance, but he was not aware of those, because they would inherently be serviced by the dealer under warranty conditions. And he rebuilds older engines back to MB spec.

PS: all of his 3.2L W112 engines have a full five year warranty and are re-built with factory parts. So, if he thought there was a problem with the overall engine or it's internals, I don't think he would be giving such a generous warranty. $7,900 for a full rebuilt W112.