Hi,
Here are my thoughts on your questions. I hope some other “collectors” out there will add to what I think.
To find out a little bit about them I suggest that you do an internet search on this machine and read the reviews. That's what I do. I read all I can on the different Singer models that I see on CL. I usually decide that I MUST have that model too! (It is an addiction).
This machine is favored by many people on this board for free motion quilting due to its vertical bobbin. I can't remark on that because I use my Bernina's to FMQ. Search this board too for this model of Singer and you will find many posts discussing this machine.
Regarding your question on if this is a good price...........hmmmm. Most folks on this board don't talk price because there are so many variables; what condition is the paint/japan and decals in, does it need re-wired, does it have the cabinet and matching chair (if you prefer a cabinet machine which I do), what accessories and extras are included in the price, what area of the country do you live in (some areas are flooded with machines and other areas have limited machines for sale). I think it is a personal decision on how much to pay for a machine. I paid too much for my first treadle because I was so anxious to get one. I don't really regret it though because it brought so much joy to my sewing room. I waited 2 years to find a 15-91 that was in the price range I was willing to pay. But, I can wait patiently since I have dozens of machines already! “Do you want to wait or do you want it now” is another question to ask yourself.
I suggest you look at the actual selling prices of 15-91's on ebay to get an idea of what machines like that are selling for. Please take into consideration if the ebay machines have been "serviced" and the wiring has been checked and is good.
I wish you happy researching (that is half of the fun!!!) and please let us know what you decide.