The original harness will be no good. Yes, the new harnesses will be free of defects....it was common problem (remedied)and even if it was replaced initially within the first few or 5 years, its probably a good idea to go new now with that item. Fixing this car up is better than buying a newer one IMO. I've seen W124 wagons with 400+K on them..of course with the $$ spent to keep them up.
It will say on the VIN sticker what month it was made/finished. Doesn't mean the parts on it weren't from a shelf from the previous model year though.
Where are you? Pix? Someone here will take it off your hands I'm sure
These are great cars still, even the sick ones.