I bought a 15-91 a few weeks ago, and found an online copy of a manual here:
http://www.ismacs.net/singer/manuals/15-91.pdf while doing my research
Your machine does appear to be a class 15 machine. The feed dogs will drop for free motion work, but you will probably need the manual to find the correct screw to loosen on the underside. It takes standard 15 class needles, like Schmetz, and low shank presser feet. I've only bought the feet for FMQ, so I haven't had to worry about making sure they work with the more narrowly spaced feed dogs of a straight stitch machine. The bobbins are easy to find, also called class 15.
Have fun sewing on your "new" machine!
Elizabeth