I know a guy who has over 480,000 miles (albeit mainly highway travel) on his '84 617 turbo,
same engine, turbo, pump...the head has never even been removed. He religiously dumps the oil every 3000
(uses Castrol 20w-50), but is a bit hit and miss with the other maintenance. It runs a little weak until it
warms up, and the turbo is a tad noisy, but otherwise he still bombs it at 90 mph during the daily
commute...I keep telling him he should at least replace the timing chain...

I think this is probably the exception rather than the rule tho, most people don't keep up on the
maintenance, or they get overheated and at least need a head gasket.

From what I can gather, the 603's are "as a rule" equally reliable, as long as you cater to their quirks
and don't ever overheat them. The 350's seem to have some special issues, but I think that's mainly
due to premature head gasket failure (water doesn't compress, which over time can bend rods yada yada).
Actually my next Benz will probably be a 350...or maybe a 124 E400/420E...can't yet decide.