What year is the car? Is it an SE or SEL?

If it is an SEL, it could be the O2 sensor. I experienced a similar problem and replacing the o2 sensor which solved the problems of difficult start, stall after start, and rough cold idle.

It is odd that you smell fuel when the car is running. There should be no fuel smell in the passenger or engine compartments. I recently smelled fuel in my 1989 420 SEL. The problem was the hose from the cooler to the fuel distributor. It was leaking where rubber hose meets the metal fitting. The fuel was not dripping. I found it by using new paper towel all around the hose and then smelling them. There just a slight film of fuel on the fitting and hose, which evporated in the hot engine compartment.

I recommend checking all the fuel lines for a leak.

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1989 420 SEL