Hard starting when hot is probably caused by a leak from the high pressure side of the fuel system to the low. When pressure is lost in a hot engine, fuel vaporizes and you have to crank enough to get all the vapor out before you get to liquid fuel.
Places to look: fuel pump check valve, accumulator, warm-up compensator, fuel distributor presssure regulator, cold start injector, fuel injector, and a couple of other places in the system. There are rubber seals and diaphrams in most of these that get old and leak. The MB manual has a diagonstic routine for finding the problem, some requiring a fuel pressure guage. Of course, you have checked ignition and idle mix.
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has a fuel pressure guage # TA 33865for $80.41 that fits Mercedes. (Seems high but has a variety ofadaptors for metric and other vehicles - was $69.72 a year and a halfago.)