Thats a common problem with the 300 and its not common for the water to end up in the oil.. Most always oil in the cooling water... When you have the work done by who ever be sure to get the gasket from Mercedes.. They know they have a problem and have made some changes to the gasket.. Some aftermarket ones are made like the first ones and will not hold up..
Its not an easy job.. in general.. you need to unhook the timing chain.. and all the stuff from the intake and exhaust.. They lift with the head.. So you need some type of lifting device.. its heavy with the intake and exhaust on the head.. Not an easy job.. but not a real hard one ...
I have no idea as to cost.. I have done one but did not even try to keep track of the time.. and spent lots of time checking every thing.. If I had been doing it for wages I would have lost my job... :-)