I'm afraid I really can't answer your questions with certainty because I don't own one.
I can say that I believe a 600SL is an awesome one and I covet one greatly. I think I have broken one of the ten commandments...
I can only go with what I've read on this site, and I don't believe they are materially more expensive to own than a 500 SL.
I gather there might be some added costs because of the two extra cylinders, and some repairs might be more expensive because there is less room to work in under the hood. Kind of like the same issues the W124's have with the V8's.
But then again, the R129's were designed to accept these engines, so it may be different. It might be tight to work on even with a V8, though.
I guess my advice is to make sure the car has a full maintenance history, and to get a good warranty - not because it's a V12, but because it's always a good idea to get a warranty on a used Mercedes, whenever possible.
Here is a link to the MBUSA home page. It doesn't go back to 1993. It starts at 1995, but it's really the same car in your case:
Have you tried e-mailing Ashman? He's a great guy and could give you a ton of first hand experience on this car.