You should switch your oil change interval. Long drives are much easier on the car, and you could add a thousand miles if you're on the highway. Shorter trips are more demanding, and under those circumstances you should shorten the interval. However, that said, in your case it makes no difference as 4000 miles is still plenty short.
100,000 miles is a short distance in the life of the M104 engine. You may require a head gasket sometime in the future, but that is probably the only driveline attention for at least another 100,000 miles. Take care of the tranny (change the fluid/filter every 30K-miles if it's the four speed, I think a very limited number of 96's had the five speed) and it'll last another 100K or more.
Change the final drive oil every 50-60K miles and the car will be in the junk yard long before that gearset gives out.
Keep on all the other assorted fluids/filters and all the related systems will last years and years to come. I have heard some early W210's have steering rack problems, but I would think by 100K that would rear it's head. Any techs want to wade in on that one?
The W210's also have some issues with front suspension components, including lower front ball joints and control arm bushings. The remainder of suspension bits last a long time. Shocks are very high quality and will last for many more miles.
Overall, you're still at an early point in the car's life. My old 190E's were not as well made as your E320, and I sold them at 250K-miles, still running, but in need of work. I see no reason why you couldn't beat that mileage by some margin.
1998 C230 "Black Betty"