If you can't locate the fuel leak in the engine compartment you should check around the fuel pumps at the rear of the car. The crush seal between the check valve and the pump are bad about leaking, which could be causing a lose of fuel pressure and difficulty starting.
The original crush seals were a thick copper seal which is replaced with a thin seal. Part #007 603 010 112. Use great caution if you are going to attempt to reseal the pumps yourself.( Strong possibility of massive fuel spill.!!!)

Also you say you checked the fuel lines and pressure regulator on the fuel rail, did you pull the vacuum/bleed line off of the regulator and check for fuel there? There should be NO fuel in the vacuum/bleed hose, if there is then the regulator is leaking and needs to be replaced.

Good luck, Robert

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Mercedes Benz Master Tech - Retired