Heres what I did
I'm no Tech expert by any stretch but here's what I did on my 91 190e 2.6.
As a preface, my benz (just bot) had the bright green stuff and had a lot of brown rust particles suspended in the coolant. It seemed appropriate to do a full fledged flushing of the system.
1. With cool engine, drain coolant at rad and engine block plugs. I have one engine block drain plug which is a bear to get to even under the car on your back. Closed plugs.
2. Removed the thermostat and reinstalled the tstat housing cover.
3. Filled with water thru the expansion tank. Put on tank cap.
4. Ran the engine for 10 minutes with heater on and fan on high.
5. Waited till next morning, guareenteeing engine was cold and repeated step 1.
6. Reinstalled t-stat.
7. This time added Zerex Super Flush and filled with water thru the expansion tank.
8. Ran the car for about 5 days totalling about 6 hours normal driving. Checked the expansion tank every morning and added water as necessary to fill line.
9. Drained car again at rad and closed plug.
10. Installed Prestone flush kit unto heater inlet hose and flushed system as per instructions out thru expansion tank until water was clear.
11. Drained rad yet again, closed plug.
12. Installed new t-stat and upper heater hose (as original felt mushy).
13. Filled with Zerex G-05, took like 3 qts. Installed new expansion tank cap. Ran engine for 10 minutes with heater on high again. Let car cool for about 3 hours and topped off with more Zerex.
I checked the coolant level for the first few days afterword and added as needed - I didn't have to add much.
I recommend new tstat and cap - both mine were suspect and using either Zerex G-05 or the Mercedes coolant.