I have 2000 E320 wagon. The car looks reliable, but I think that biggest issue for this model is rust. I have fixed rusted spots on trunk lid and one more came recently under trunk lid window seal (I guess that better solution would be replacing whole trunk lid panel). On last weekend I replaced front springs and found rust spot in spring perch/top mount. It is kind of dangerous location and I'm going to fix it ASAP.
For unknown reason Mercedes zincifying only 75% body and cover it by warranty only for 4 years/50 000 miles.
I would advice before buying S210 very carefully check car's body on rust spots. If you are DIY'er then all other problems will be looks like nothing in comparison with body works.
Most popular rust places are spring perches/top mounts, trunk lid, door window frames (check it under seal) and door's sheet metals under trim.