I don't have as much experience with the W140's, but I know that sounds very normal for a W124 4.2 V8. If it stays above 100C while cruising at highway speeds-worry, if the temp gets above 110-115 while sitting-worry, what you describe sounds normal to me. While sitting the electric fans should kick on when it gets above 100C or thereabouts. A/C usage will tend to raise engine temp, but not in such a way that you should be concerned. From your description, it sounds like things are as they should be.
Hope this helps...Lee