Hi,
For the best qualified answers to your questions regarding the 6.3 you might want to check the forum
of the M100 club here:
http://www.m-100.cc/forum/default.asp This club covers all M100 powered cars including the 6.3 of course, but since you are looking for a 6.3 I'm sure you already know the site.
I have owned several of these cars over the years and still have two 6.3s(and a parts car), they are a lot of fun but can bring some challenges as well.
For a potential buyer of a 6.3 there's of course as with every old car enough to worry about. As with all 108/109 cars the rust is often a serious problem. Mechanically the 6.3 is a bit more challenging than most other MBs from the era except for the 600.
I'd say the most important thing is finding a car with little of no rust that has been well maintained and not been sitting for longer periods of time. And if the suspension still is in original shape be prepared to do the suspension valves, air hoses/lines and airbags rather soon than late.
The air compressor is pretty robust and does not give problems too often. Airbags are available and the suspension valves can be overhauled(Star Motors in NY is one place to go for this). Once the suspension is done, it will last for many years and give a superior driving experience compared to even more modern cars.
Engine work can be very expensive, pistons are quite expensive and it can be a problem get the right ones, since there are a number of variations (hi/lo compression US/Euro specs etc) Injectors are insanely expensive but can be rebuilt same for the Fuel IP. Transmissions have a tendency to leak both externally and internally(check for delays when engaging in position Drive), and the rear axle have some grease points that sometimes are neglected and can cause excessive wear.
In short find a good well maintained car with history and service records, without a long backlog and spend as much as you can upfront.
Regards,
Hans