Replace the thermostat. Period.
Then, if the idle remains abnormal after the engine temperature is
correct - 80 or 87 degrees C. , whatever - then focus on the idle. The
oil pressure is OK at 1 BAR when idling, not a problem.
What is M-B's idle speed specification for this engine?
Let's not mess about trying to achieve something that's not specification!
I once owned one of these and vaguely recall it idled quietly and slowly
- or it seemed that way after owning a diesel!
CP, always, always first look at the simple things for they're not only
cheaper to fix but usually THE problem's cause.
Finally, change the oil, do the required maintenance and don't look for
trouble. These cars are designed to run for long periods.