You might want to check the rearend gear ratio on the car. Most of the petro engined Mercedes that I've had experience with have lower gear ratios than the diesels. A normal 240D in the U.S. has a gear ratio of 3.46 whereas the 280CEs have a gear ratio of 3.69. I don't know about 230s.
Yours could possibly have a much lower gear and the diesel just doesn't have the power to push it above 70.
Check the gear ratio by looking at the left hand, lower back edge of the differential, there's a flat spot there and the ratio is stamped into the metal. You generally need to scrape off the dirt and grease to see it.
My daughter's 240D will do well over 70 but is very wound up above that. Hope this helps.