I'm glad the Tstat and belt are taken care of.
I honestly don't think it could be anything other than a water pump at this point. How much coolant will it leak out after being driven? Does it leak at the front of the car? Take it out for a drive and park it, pop the hood and watch carefully at what unfolds. Watch the fittings and make sure they are all tight. Even if your grandfather replaced the pump just a few years ago, it could have certainly failed by now. I have seen stranger things.
A new water pump @ the dealer for your car would likely be $650-750+ installed as the part alone has a list of $368!! It is not really an easy part to get to on your car, kind of a pain in the butt.
The M103 engine also has a habit of blowing head gaskets. Do you know if your car has had this service performed? I know about this problem from experience
Be careful not to over heat your car as it may damage the head.
Your oil pressure sounds normal to be.
These cars cost money to run. Sometimes a lot of money. One repair can essentially "total" the car. I would kill to have my grandfathers old Benz, what a cool thing to pass down.
Good Luck,
-GH