Thanks so much, folks, for the help and advice. I did replace the antifreeze and added a bottle of water wetter to boot.
A word of warning: I replaced my leaking overflow tank as well, and the new one had a hole that my other one did not! The counter clerk at the MB dealership gave it to me, and I had the foresight to compare it to the one on the car, as something didn't look right. I brought the new one back in to the parts dept. and began telling the clerks that I thought that the one they had sold me was for a different year. The other parts clerk assured me that it was for the W123s, but that the other clerk had failed to give me a little kit to plug up the extra hole. Once we got that in ( a little work) we were good to go. Thanks so much again for your help and advice!