Thank you very much for all the info.
I think this car will be getting a Sanden replacement compressor, heater box clean, scrub, reseal, and recharge with R12.
I will take it slow and replace all of the vacuum equipment under the dash as needed as well. No need for you to count, I think that I had suspected three diaphragms as well.
How right you are about the dusty desert. I believe I should have a few quarts of dust and sand in the heater.
After trying to replace the thermostat on my '99 E300, I'm very frustrated
and think that the old '77 is the keeper out of the two.