Tom and Aaron just gave you advice that is priceless. Expect various leaks. Drive the junker though. You can tell by feel if it's OK. The benifits of a parts car are huge. Extra, probably shot tranny, but who cares. An extra and probably good rear diff, that's a 2500 $ part.
Go into this slowly, make sure, YOU ARE SURE. A nice 6.3 is about the finest auto in the world and a treat comes with every drive. I just spent a fortune stopping tranny leaks. Quess what, it still leaks. Oh well, tough, right? Oh, but the power. This one is like magic. A monster with a perfect engine, near perfect tranny, rear diff, perfect interior and pristine paint.
Don't be afraid to change interior leather colors nor the paint color. Benz designers had a terrible paint color team. Muddy colors. Of the originals, I accept only five as decent; white, black, silver, silver blue, and the dark blue. Otherwise, I change them. Drive the color you wish type of deal. You can even change the woods. It is possible. Cognac with burl, pristine, again. What a combo with a dark, dark, green.