I have a 300-SEL - 1991 - W126 body.
I had to pull mine last week to replace a battery. It's not hard.
The reservoir is not fastened by bolts, etc., it simply lays in the area in front of the battery.
There are 2 coolant lines that come off and 3 elect. connections. One elect. connection is on top, about in the middle towards the back. The other two are down below, in between the battery and the reservoir.
If your reservoir is full, it might help to first siphon out the contents. This is an easy job.