One of the main reasons I like this board so much is that I know so little about cars, and find it endlessly interesting to read the advice of the guru's of this board.
I have an '87 260E with 280K+ miles and a '90 300E with about 103K miles. The one "big ticket" item I had was the a/c r134a(?) conversion, about $1000USD on the '87. It has about 150K on the first valve job, so it may need one soon, also about $1200USD. Doesn't use any oil yet, so we'll see how far it will go.
The 300E hasn't had anything "big" yet. It had the SRS light coming on, and cost less than $100 to fix. These are my daily drivers, about 30-35K per year combined. So if I were to figure the per mile cost for me, it would quite reasonable. Reasonable for even an American car, but simply a delight for a Mercedes.
I can't add anything that hasn't already been said. Drive it, enjoy it, be proactive on maintenance and the car will be very good to you. My experience has seen a reliable car that is a joy to drive.