That BMW parts looks like the simplest approach, though pricey. There are several other options. One is that someone packages a kit from later M-B parts which has a vacuum on-off valve and electric vacuum switch to actuate it. Similarly, many U.S. cars had vacuum-operated heater valves, many spring-loaded so vacuum to open (or vice-versa). I have a $7 plastic one for a Ford (for my classic cars, but could use on my W123). Just get the polarity right: 12 V on (from push-button box) stops the flow.

Another approach is to install a W124 valve. You have ample room if you rip out the silly "aux water pump" to install it at the front there. Extend the control wire and add a section of 5/8" hose where the monovalve sat. One guy here did this. I have a W124 valve in case I run out of W123 parts. My 2 300D's don't have the aux pump and I get plenty of heat.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans