I have had my 400E for 7 years and continue to admire and love it more all the time. Lots of power and refinement.
Of all the maintenance/repair issues brought up, the most expensive is the AC. Many W124s will eventually need evaporator replacement (mine did 3 or 4 years ago) to the tune of $2000 or so if you have it done.
The second biggest repair expense is probably the throttle actuator. Probably $1100 or so.
Wiring harness is not quite so bad -- $450 for parts, a couple hours labor, and not bad as a DIY procedure, I'm told.
Be aware that the M119 engine in a W124 body is a TIGHT fit, so virtually every job under the hood takes longer...