I am seriously overwhelmed by the number and thoughtfulness of your reponses. Thank all of you. I intend to consider all of your experienced opinions as I continue my education and search.
Do you know of a site that describes the various models by type designation (123, 107, etc) with corresponding engine and body styles?
What are some simple observations to be made upon viewing and driving an example of, say, a 240D or a 300D? I have looked around this site and read about discoloration of fluids, noises from differentials and so on. Any other features on a cursory look that could be noticed prior to taking a prospective car to a mechanic?