Great buy. First of all I'd lubricate under the machine where the feed dogs live. Some Singers have a release (wing nut?) under the base to release the fed dogs for FMQ. Some one else who has used it can better describe this.. Or a manual if you have one.
15-91s are potted motors, not belt drive like yours. Other variations are smaller .
I am surprised by your base. Looks like there is no room for a top case.