Is it safe?
 

Is it safe to put the engine flush before you do your own oil change, because theres a bit sludge on the dip stick and the oil cap.

Engine flush?
 

I don't see any profile information on what year ML you have, but if it's always had synthetic oil, there's no reason to use a flush. The brownish stuff under the oil cap is normal condensation. Not sure what could be on the dipstick, unless you use "Dino" oil.