Ali Is right,steer clear of the 6.3...we need to keep the value down So i can buy them... .
There is only one way to buy a W109-018(6.3) ,get an independent assesment done by a MERCEDES specialist,and buy the best you can afford.A two owner 6.3 has been sold recently in the US for $95,000.That's how much you will have to pay to get a car in "turnkey" conditon,$25,000 gets you a useable good conditon car and anything less is really just a project.
Good W109 3.5's will get $10,000 + and the same rule applies ,you pay less- you get less.