You make some good, and imo correct general observations. Firstly though, I would say from my MB experience, that either chassis is plenty robust-enough to handle the roads you're describing - these are two the most revered modern designs for longevity and build quality from MB.
While the W124 is thoroughly more modern and higher performance in every way (except wieght
), I would say the W123's while older, would still be less expensive to maintain. I don't hear of evap core leaks on the W123's that one does on the W124's. Also, there have been mixed reviews on the reliablity of the later generation of W124 diesel engines, wheras the 617 motor family is proven without a doubt to be very bombproof.
For examples in similar condition, you'll still spend more that double for a W124, so I would say both budget ongoing maintanence and for the intial purchase price should be considerations. I'm not saying the W124 would be a money pit by any means, but the lack of electronics and more cast iron on the W123's makes them simplier machines.
...and that's what they drive like - machines. Like I said before, the W123 lacks the sophistication of the W124, but has a certain feeling that many, including myself appreciate. Has the grunt of an ox, but still impressively refined for a 20+ year-old 5-cylinder diesel car. Cruises on the freeway with that typical MB composure, but can feel like a bit of a pig around town.