What year and model W115 is the engine going into?

If the car was a 4 cylinder diesel perhaps you could use the gorilla knob to control your glow system and start it, just like it was with the original engine. If the car was a gasser you would need to come up with a way to control glow and then use the key to start. Full manual control of glow with a button on the dash or console might be simplest.

For stopping the engine the ignition switch from neither gasser nor 4 cyl. diesel has a integral vacuum switch so using it to control a vacuum signal to the shut off valve on the IP is out. But either ignition switch could possibly be used to send 12V to a solenoid valve that prevents vacuum passing when energized, when the key is in "run" position.

If the car was a 5 cylinder 300D then you probably have all this covered with the existing ignition switch that's in the dash.

The only engine sensor the car is looking for is the temp sensor in the cylinder head, used for the temp gauge in the cluster (could be different for W115 300D). The '73 W115 I had for years had a non-electric old school gauge with a probe that went from the cluster to the cylinder head. The '74 I have now has a electric temp gauge setup, and maybe the later electric sending unit like in your '84 engine would work properly connected to the later style W115 electric gauge. If not swapping the sending unit from the original engine would solve that problem.