I don't know if you have seen these 2 threads on the m103 and m104 engine, but there are some valuable opinions in these threads from Techs.
THE M103 ENGINE;...... GOOD OR GARBAGE?
104 eng. Bullit proof? or Prone to breaking down?
I believe the wiring harness issues started with the early 90 cars - I have not seen any late 80s w124 car owners discussing a wiring harness issue. AC r12 conversion issues, but not wiring. Unless you have a 4matic or 500E or TE with air suspension, that major potential cost is a non issue as well.
Also, the regular theme through the last 2 years on this board seems to be that the w124 cars are among the most reliable of the gasoline Benzes, and w123 diesels the reliability kings.
This Federal site on technical service bulletins is very useful as well. http://www.nhtsa.dot.gov/cars/problems/tsb/Index.cfm
Good luck on your quest.
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
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