I still have the same problem but after check many different points as suggested, I found out that the solenoid at the starter is not working properly and also the carbon brushes of the starter need to be changed.
Another test that I did to make sure that the problem was not from other connection point, with engine hood open, using a remote start or a piece of wire, I jumped terminals 30 and 50 on the connection box into the engine compartment.It's a quick test.
In any Mercedes book you will find where is located terminals 30 and 50.
About the Alternator the suggested test is the same given on the previous reply. I did the same way and it works OK. Most auto parts can test it for free.
There is a need of a bench test when alternator is removed from car.
Hope it helps you.

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Nyck - TX - USA