As I understand, the v-8s had some wiring harness problems, as opposite to the 6-cylinder models, which presented a lot of them.
W140s were VERY unreliable until 1994 (it took M.B. more than four years to sort out most (not ALL) of the problems that the electronic monster-mobile presented. Late production 1991, 1992 and early 1993 are the worst.
Actually, your story seems half the horror tale from what I'd heard from other early production W140 owner's.
Post 1994/5 models (more or less around the time of the facelift period) are usually more reliable but definitely not bulletproof.
CL 500, 1998
S 500 L, 1998
E 320 T, 1995