Yeah, the M104 engines are known for head gasket leaks - oil leaking at the right rear of the gasket, or at the front at the timing chain cover.
Many cars, including mine, have experienced engine fault codes suggesting low egr flow. The average mechanic will want to change the EGR valve, thinking it is stuck closed. Two separate MB Independent mechanics that I went to suggested this. Stevebfl posted an article on troubleshooting engine diagnostic codes, and in that article, discussed the problem of carbon coking up the hot inlet pipe to the intake manifold.
This "hot inlet pipe" gets blocked up and the fix is to ream it out with a speedometer cable and a drill motor.
I've read a post from Benzmac that his shop performs this function every 30,000 miles on the M104 engine.
Last, the engine wiring harness was not a problem in 1991, or 1992 for that matter. It is limited to 1993 through 1995 models.
You may do a search and read hundreds upon hundreds of posts on this.
I'm sure you'll enjoy your 300CE. It's one of the most beautiful MB's designed and, I believe at some time it should stop depreciating and perhaps go up in value due to the relatively low numbers that were sold in the U.S.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".