HI oceanrider!
I am currently restoring a Nishiki Sport Special. I believe it is from the mid to late '70s. It has a chro-mo frame, with some really beautiful lug work. With a frame wieght of 6 pounds, it is a tad on the heavy side, but should yield a very comfortable riding bike when finished.
Nishiki at one time made some very fine bikes. I recently attended a triathalon, and saw that one of the riders was riding a Nishiki.
As far as the Suntour components, they are fine. My first road bike, a Fuji Roubaix, had a Suntour GPX groupo, and it performed flawlessly. Suntour simply got out-marketed by Shimano, and quit making the road groupo, but continued in the mountain bike line. I have heard rumors that Suntour was going to re-introduce thier road bike line, but so far, have not seen anything.
For what might be a good frame with a decent groupo, and with the warranty and service, the price doesn't seem out of line to me. Shoot, if it fits, ride it! You'll have a blast!