The Klima is an interesting question. Is there a Klima relay in a 123? I don't believe so. If simplifying, it can go, as it's primary function is to keep the belt from shredding if the a/c clutch seizes. In my experience, it mostly malfunctions due to the belt slipping. The Klima relay also powers the kickdown solenoid on 124s'. I think there's a different power source in a 123.

I see no point in eliminating either the Bufo or the overboost switchover, but you certainly could.

The tach won't read correctly, as it's set up for a five cylinder motor. You may be able to transplant the guts from the '87 tach, this would be a project in it's own right.

You will also need the EGR transducer.