No idea what is needed for the swap. I owned several 123's and I cannot fathom why you would want to invest a few hundred hours into this project, when you can go buy a complete running 400E/E420 in good condition for ~$3k, complete with antilock brakes, a real suspension front & rear, and a pile of other improvements. Seriously, the ROI on this upgrade would be terrible. If you've never owned a W124, I strongly recommend you go test drive several (to make sure you don't drive one turd, and think they are all like that).
Anyway: you would need access to the complete donor car to harvest the wiring harnesses, CAN box, modules, etc. Standalone ECU might make things easier, or could be harder, I'm really not sure. I don't believe this swap has been done before, so I don't have any links. Maybe see if you can find a W123+M117 swap, as the M119 would be very similar. If you attempt this, I'd do it with a 5L engine, unless the goal is fuel economy.
