Some light reading...
Description of operation of the fuel system
http://w124-zone.com/downloads/MB%20CD/W124/w124CD1/Program/Engine/602_603/07.1-0010.pdf
Wiring diagram for the 124
24 Electrical Troubleshooting
I would carefully extract the entire wiring harness from the engine when you pull it. The engine harness joins to the cockpit harness within the fuse box. You may have to do some gymnastics to mate the correct signals coming from your 123 cockpit so I would plan on getting the cockpit half of that harness too.
There are several engine systems you will need that are different than the 123.
- air conditioning system will need to be integrated into the engine controls. The 617 has a direct belt to the compressor but the 603 has a serpentine belt. The engine RPM sensor and the Klima relay are necessary to protect the engine from a seized compressor. They tie into the tach sensor in the compressor to determine if the compressor is rotating slower than the engine.
- the overboost protection is part of the engine harness. It doesn't rely on the ECU but you will need it to protect the engine. The manifold port plumbs through a switch over valve into the ALDA and the blue flying saucer for transmission control.
- the EGR/ARF are controlled by the ECU. Your choice if you are going to run it or not. If you don't then you will be able to simplify the control system by deleting the vacuum transducers at the right front of the engine compartment and the attendant plumbing.
- turbo waste gate is direct controlled in the 87, unlike the later models that rely on the ECU so that is easy.
- tach is derived from the crank sensor. The 603 sensor is 144 pulses/rev (on the ring gear) and the 123 is 1 pulse/rev (on the balancer). You will probably have to pull the tach mechanism from the 124 cluster and figure out how to install it in the 123 cluster.
- oil pressure is a mechanical gauge in the 617, 603 uses an electrical sensor. You may be able to adapt the bung hole in the 603 filter tower to accept the 617 gauge.
- not sure if the temperature gauges are compatible between the 617 and the 603. The sensors may be different.
I would also locate the diagram for a 123 so you can look at integrating the start, glow, alternator, temperature gauge, etc.
If you run into more specific questions let me know. I deal with control systems at work and I'm used to figuring out how to make things that weren't designed to work together play nice.