I had the same problem on my 300se and changed lots of parts,
and did tune up distributor cap,wires ,sparkplugs and no difference after 3 months I tried techron fuel injection cleaner and
the car would start when hot first try (I felt like im in heaven).
As soon as I refield the tank the problem came back then I thought as long there was fuel injection cleaner for like two weeks the car started hot first try with no problem. Then I decided to take it to a mercedes shop and told them my story what's going on they said the solution to my problem is to force some kind of special cleaning solution through the injectors from top of the rail where the injectors get fuel not thru the gas tank. I paid $119.00 in Ohio and that completely eliminated the hot start problem and its going on three months since the dealer cleaned them.
Before you spend your hard earned money try a bottle of good fuel injection cleaner (always good idea to change oil after)
if the problem goes away (cheap way) then take it to a shop so that they can clean em thru the fuel injection rails(MoreExpensive). Good luck