I almost bought a 15-91 like that. They are relatively rare, because they have the nickel stitch lever cover plate, and the Singer emblem is nickel as well.
$50 is a great price for a regular 15-91 in a cabinet, so for that model, it is a really great price. If the wiring needs replaced, and you can do it yourself (it isn't very hard to do), that could run $20 or so.