pretty normal...I'd top up the coolant reservoir with water the next time I came out in the morning before I started up...likely the coolant "loss" won't even be noticed.

Ironic that the times you want to run AC the most are the times it's hardest on the car.

You should be able to find an owner's manual for the W123 diesels at a junkyard, used car lot, or from a mechanic who works on a lot of these cars...I'm amazed at the number of these cars I find junked out but with manuals, operating tags for various controls, and even the booklet on where you could find diesel fuel in 1983 still in the glovebox...ask and you can likely be told just to take it.