My newest wagon ('94) is at 210K, having bought it at the beginning of the year. Despite the mileage, it runs like a new W124, even better than my '95 wagon with only 150K.
Just as with any other car, especially a MB, make sure that it has documented service history and that the nagging stuff has already been addressed such as the wiring harness, ETA, headgasket, etc. Hopefully you're lucky and find one with all the expensive repair stuff like AC evap core and HVAC vacuum pods already done.
Do expect the car you're looking to either have a rebuilt trans or a getting tired original, again depending on how often the fluids & filter were done and driving style.
I bought mine off CL for $3.5K but with close to 20K in documented service history. Even so, it still needed some things like cooling system overhaul, brake overhaul, engine & trans & diff mounts, new steering lock assembly, fuse upgrade, etc, etc BUT the big expensive stuff was paid for by the previous owner.
Don't fall in love with the first W124 you find. Apart from rust, mechanical condition is the most important factor. Pass on any MB or other car that has any signs of rust (bubbling paint on the fender, for example) or worse rusted through (perforation). Too expensive to fix and not worth it.