Whoops! I didn't see this thread here when I posted mine last night.
viewtopic.php?f=1&t=272474
From what I've read here at RB, the early full sussers designed by Richard Cunningham (specifically the Mantis Pro Floaters and Nishiki FS bikes) were very good designs, but I haven't seen anything regarding restoration or maintenance costs. From some of the reviews I've read, those old RC bikes will put a smile on your face, and will handle moderate trail riding with ease, but I doubt you'd want to hammer them as the suspension travel on those old rides is not nearly as long as that of the new ones.
One thing's for sure, like most retrobikes that are maintained or restored, those old suspension designs look damn cool, and folks that see you ride them will recognize you to be the bike connoiseur that you are, instead of just someone that bought whatever your LBS wanted to unload on you to make room for next year's models. Of course, if your modern ride happens to be the latest top-of-the-line full-on race version, then you don't care what anyone else thinks because you know that you have the best that technology has to offer, at least for now. That's how I felt about my 1989 Alien until about...er...1990.
Of course, now that my Alien is 'retro', it's cool once again.