There was a good article some months back in the Star Magazine regarding the bottom end of modern MB engines. In the article, their claim was that unless the engine is driven fast alot, such as on the Autobahn, the bottom could be good for a couple of head rebuilds.
This is interesting, but as you eluded to early in this thread, the maintenance attitude will play heavily into this.
Last summer I did the cylinder head on my '88 103 engine. As I recall, the engine had about 175K miles or so, and there was no ridge that you could feel with your fingernail and there was prominent crosshatch. The engine turned out great after I got it back together.
With YOUR maintenance regimen, I expect your engine to live somewhere close to "forever".
Have a great day,