Mercedes ML320 (w136) starter motor issue..?

It sounds like a plugged or busted fallopian tube. Look for a metal tube going from the starter to the area above your oil filter. It provides lubrication to the throwout bearing in the starter. Disconnect the tube at both ends and spray some carburetor cleaner through it. If it's busted a new one can be had for about forty dollars from autozone. Likely one as old as yours will need to be special ordered