I'm not an expert since I have limited exposure to the 124 and 210 cars (too small ) and my 140's been well behaved... touch wood.

The summary of what I've read is that the 124 E320 is a relatively troublefree car beyond the wiring harness and head gasket issues which seem to require one time or at worst infrequent attention. Replacing the AC evaporator is a big job but not as big a job as on the 140 and without the same history of recurrence.

Little things go wrong with the early 210 E320 and there's head gasket issues as well but I don't recall reading of AC evaporator or wiring harness issues. Well worth the price premium and new model bugs if handling matters to you.

The S320 had the 5-speed transmission from day 1 (I believe) so the chassis makes more efficient use of the engine's output. It's not a match for the V8s but it can hustle when called upon to do so. I've read that a 140 can nickel and dime you to death if you're not a DIYer. It will quarter and half dollar you to death if you pick the wrong one. In general the 140's biggest bite is the AC evaporator which is a $2500-3000 job and some have indicated at least one repeat job. The door closing assist function can get flaky which is more a problem for the trunk than the doors since typical closing force will close a door properly whereas you really have to slam the trunk lid to skip the closing assist part. Rear power headrests (the fully motorized kind, not the flop over kind) can stop working, the power rear view mirror (through 95) can get loose and/or the adjustment control can quit, etc.

Sixto
95 S420
87 300SDL
83 300SD ... $950, see cars forum