Some refere to the W123 as old, out dated and clunky, and the W124 as modern styling, smooth, better ride and all the newer up grades, and quieter etc...
the OM617 has the cast iron head, and can take more abuse some what over the OM60x with the aluminium heads.
These are old cars that will need some tinkering to stay on top of the maintenance, and probably will be repairing past PO`s defered maintenance and abuses.
The newest W123 is 27 yrs old and the 87 W124 diesel wagon is 25 yrs old.
If you do not want to do your own wrenching, it will be costly to pay the hourly shop price. learn to be a DIY`er and your wife won`t have trouble knowing where you are.
Here is a W124 buyiers guide
1992 300D Turbo Diesel Recommendation
Charlie