Dave,
When I have the same starter problem with the 240D I tap the side of the starter kinda hard with my trusty MB lug wrench. Never fails to work for me. Problem is my starter will NEVER fail at the shop for a diagnosis & solution to the problem.
You might make sure the Battery terminals are clean and tight and that the ground strap connections are clean and tight.
Hope this helps. Hope you can make it to the shindig after all. The first one was really fun.
Good luck!

__________________
They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety. Benjamin Franklin, A.D. 1759

Roger
1975 240D
1986 560SL