I think the more practical alternative to a 6.3 is a 300SEL 4.5. The 300 has more wood, leather standard and other nice touches. They always seem to be floor shift as well. 4.5 parts and service are easier than the 6.3, although the M100 engine itself is quite durable. The transmission in a 4.5 is going to more reliable as well - not overmatched by the engine. The 6.3 is a 4 speed, and then they went back to a 3 speed on the 6.9. I once had a 6.3 with a 6.9 transmission, because the 6.3 transmission just wouldn't hold up for the previous owner.

A 300SEL 4.5 is a great car - fancier than a 280, and still pretty easy to take care of. Another great thing about it is that you can convert it to coil springs with some 108 4.5 parts, if you get tired of taking care of the air susupension. I've never heard of a successful coil spring conversion on a 6.3 - the engine just weighs too much. A 300SEL 4.5 with coil springs is a very practical and desirable classic car - assuming your not too hung up on originality.