I can't say whether your cars have the wiring harness problems. But, the M104 2.8L engine does suffer from head gasket problems, and that seems "world-wide."
The M111 DOHC four cylinder is pretty bullet-proof, and I have heard of only a few head gasket leaks, nothing like the M104 six. I know a handfull of C220/230 owners with very high miles (350,000+ kilometers) and have had no engine problems.
1998 C230 "Black Betty"