Singer 66s use easy to get Class 66 bobbins, easier to get 15x1 needles, and low shank feet and accessories.
Wiring sets can be had from your local SMG or Sew-Classic. Or usually you can rewire it yourself.
Manuals can be had on line, not that there's anything complicated about Singer 66s.
I've never paid that much for a 66-anything, but then I've been lucky. If you want it and are willing to give it the TLC it needs then $50.00 isn't too bad.
Joe