The dark oil, after just a few miles is completely normal. That would be the soot. That is why it is very important to have an oil that can suspend this. Even the newest diesels from GM call for 15W-40 oil. If there is Walmart near you, you can find both Chevron Delo 400 or Shell Rotella. At Walmarts here in NJ, Delo 400 cost $6.12 a gal. Remember you will need 2 gals for each oil change.
As for the coolant issue and where to buy MB coolant, I bought mine on line from an outfit called GAPA,
www.gapa.com. It's alot cheaper than what a dealer wants. There has been talk in the past about the infamous green stuff possibly causing the plastic in the W123 and later cars' radiators, to become brittle. Since I was able to get the MB stuff for only a couple of bucks more than the green stuff, I bought it. I figure it's a cheap price for a peace of mind.