I couldn't agree more about other MB models and other cars being criminal in terms of repair costs. On the Ritter/Easley list there is a W140 owner who just found out that his AC evaporator needs to be replaced to the tune of $700+ for the part and 25 hours labor. All I'm saying is that my experience and that of the 123 owners I know differs widely from yours in terms of enjoying such uncommonly low cost/mile. More power to you and maybe my next 123 will be similar.
Remember, though, that the question that started this thread was, in essence, will you have to spend money on these cars and, in MY experience, the answer is yes. However, it sounds as if you and I are in complete agreement regarding the elegance of the simplicity and efficiency of our 123s, as well as the ease of many DIY repairs. My 126 will be sold long before we are without a 123, and for just the reasons you bring up.
'76 Moto Guzzi 850 Le Mans