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.

http://www.thetoolwarehouse.net has a fuel pressure guage # TA 33865
for $80.41 that fits Mercedes.  (Seems high but has a variety of
adaptors for metric and other vehicles - was $69.72 a year and a half
ago.)