Like you heard earlier, the wiring harness is a cronic problem with these cars. When I purchased my 94 E420 back in January, the wiring harness was starting to rot/crack.. etc, so I had it replaced. I found a Mercedes Dealership who would do the labor for free if I covered the part (about $500). I have heard that some dealers (Atlanta?) replace them for free.
As far as ECU goes, you should have bought a used one. I had a problem with my ECU which required a replacement, but I picked one up at a salvage yard for $225.