You are so lucky to have such a resource for vintage machines. I have a Singer 15-91 as my back-up machine and a vintage Bernina 830, which is my primary machine. I agree with the others-- if you see a Bernina, buy it.
There are some Class 15 machines made in Japan that have the same functional design as the 15-91, but often have different paint colors and interesting badges,etc. I would love to buy several of those because I could use the same accessories but could keep them out on display. You may find several of those. The Singer 15-91, Class 15 clones, and the Singer 201 have a nice sized harp for FMQ or straight line quilting. I'll be testing mine out in a few weeks on a queen sized quilt.