I think the M113 ends up being more reliable because it doesn't have to work as hard - that's why there is very little fuel economy penalty. If you drive the 2 on a daily basis, the V-6 always seems to be in a slight strain compared to the V-8.

The W210 is a pretty good successor to the W124, about what you would expect. I'm actually thinking about one myself - just waiting for the right one to come along. You can certainly find low-mileage E320's for cheaper than similar E430's right now. If I wait long enough, I might just hold out for the W211 - everything so far on the ones we see seems like its going to be a good car long-term, though I'd want the earlier ones that still have the M112 vs. the M272 (the M272 of the E350 has had some problems with countershaft gears stripping and it requires engine removal to put new shafts in, and pulling the timing chain - though we haven't had one in the shop for this yet).