Hi ww88ww and welcome to FT.

Yes, the best way to guarantee to get a MB is to rent at a MB-service station.
These SIXT stations are not allowed to give you something else than a MB, so you can be sure of that.

The SIXT station at the Arnulfstr. (München Daimler NL Mitte) is a very busy, yet very limited one talking about choice-spread.
So normally the don´t offer you several cars to chooce from, but order and deliver the car specifically for you.

Renting LDAR you can absolutely be sure to receive at leat an E-class (new modell).
For LDAR customers SIXT often grant an upgrade, so it could be as well that you might get a S- or ML-class.
I´ve never had an SIXT E-class without navigation, but you can´t be sure to get COMAND.
The idea to call the station a few days prior helps to clarify your wishes and explaining your situation in a friendly manner might help you getting an upgrade :-)

Enjoy your trip.