If you can take it to your maybe not so friendly Mercedes dealer, or any mechanic that can plug an OBD II diagnostics computer into the car and "talk" to the computer, chances are a fault code will be stored if it is the Crank Pos Sensor. I'm not sure that a failing fuel pump will throw up a code one these. Normally,, it doesnt. May cost you a few bux, but the money and aggrevation you save in trying to figure it out, maybe worth it.

Rick