The A/C is certainly the budget-buster for the 124.
Suspension and steering components can also add up quickly.
No one big item, but so many little things...
I can't say the 124 is so much worse than other cars in this regard, other than the extra moving parts compared the more common rack & pinion config, and the multilink setup in the rear.
Look closely for tire wear, steering play, and check for unstability at speed.
Oil leaks are common, but not particularly expensive to repair.
Everyone seems to agree that headgaskets are not problematic on the m103 engine, so the earlier models do have the edge there.
These are great cars. Good balance of performance, comfort, reliabily, and surprising fuel economy.
If you like to do your own repairs, the 124 is relatively straight-forward.
99k in 14 years is amazingly low, though some components age regardless of mileage.
Good luck with your search.
1986 300E 5-Speed 240k mi.