Thought I'd jump in as long as we're talking about 300SDL's. my '87 300SDL (111,000 miles) is in Houston at the mechanics due to a bad alternator - common stuff for any car. We have a small oil leak that is from a tiny crack in the timing chain cover. We also are getting the AC Manifold rebuild because of a small freon leak - I had this same AC problem in my '86 300E.
Overall we are very pleased. I'm convinced that this car will run for a long time. The black smoke, like any diesel, will build up with infrequent use OR continuous 'putting' to the school or HEB. It's important to get the car out on the road and punch it to burn off the carbon accumulation. On our trip to Houston this weekend, once I cleared out the soot, the car 'breaths' much easier and performance is greatly enhanced. I just winsh there was a way to keep the soot from building up in the first place.
We've really enjoyed the freeway driving and the size of the car; it's great when you have an 8 and 10 year old that want to 'interact' the entire trip.
As a maintenance package, I like the fact that I can work on the car myself. I buy all my filters here at the Parts Forum which give me a lot of confidence about the quality of what I'm putting in the car. Not to invoke the 'lubrication wars', but I switched to Mobil 1 oil (15-50) and use Redline Diesel catalyst from time to time. There seems to be a lot of great experience among other diesel owners and it's a nice community to be a part of.
anyway...just some random experiences.
1992 500E (82,000 miles)
1987 300SDL (105,000)