Were I interested in 4-matic, I would skip the W124 version, buy a W210, and equip it with an extended warranty that covered AWD bits for peace of mind.
Most 4-matic W210 cars will be still under factory warranty, so your chance of finding one on a dealer's lot that could be "Starmarked" would be good.
The main problem of course, is cost. Late model E-Class cars are not exactly used-car bargains.
If you deem AWD an absolute requirement and the W210 is beyond your budget, consider a Subaru or Audi. I understand they both have bulletproof AWD systems. Volvo has one too, but it's getting a poor reputation for reliability.
1998 C230 "Black Betty"