15-91's have about a 7" space. They are wonderful machines. I think $100 is a little high and $165 is definitely too high especially if they don't even know if it works or not. Look on Ebay--that's where I got one of mine and I wouldn't trade it for the world!
You can find feet for it online at various vintage sewing machine sites like
http://www.sewingmachineplus.com or
http://sew-classic.com. The feet don't usually cost very much.