One cause of slow starts after sitting overnight is a drop in the system fuel pressure. There is a fuel accumulator by the fuel pump that can fail or you could have an injector leaking down (including the cold start injector).
I had the same problem with my M103. New injectors was the solution.

Why was the fuel pressure regulator changed?

The injectors are recent but the cold start injector is original, I will check that first. Thanks. I can't remember exactly why I replaced the pressure regulator.

OVP, fuses, starter and battery fine. Fuel pressure drop overnight sounds right.

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1988 300E

Next morning, turn the ignition on a few seconds without cranking the engine so the fuel pump cycles on and off. Do that 3 or 4 times. Then see how it starts. If it now starts right up, you know the pressure bleeds down overnight.

Thanks very much for the fuel accumulator test. I will try and hook up my CIS gauge as pictured. I now remember why I replaced the FPR, fuel was visible in the clear overflow tube. It had been that way for years, replacing it made zero difference in drivability.

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1988 300E

Thank you for that Rick i to have m103 with hot start problem. And came up with injector replacment as next job .But it will start as normal in the morning .