I learned to sew on a 15-91. I have My mother's sewing machine now that she has passed. They are great machines. Mother left hers in storage for many years, so it now needs re-wiring. The decals are in good shape, but it shows the years in UN-air-conditioned storage.
I found a model 65 cabinet in oak that I thought would work out well. It was an auction item, so I think I paid $25 or $30.00 for it. It was trash. However, it had a 15-91 in it that is in great condition.
I see lots of 15-91's on craigslist and offerup. They are asking in the hundreds, but they don't bring that. I would suggest you keep looking and pick up a 15-91 at $50 to $75. Pay attention to those items that have been for sale for three of more weeks. You can often get one in a nice cabinet for less than $100.
Generally speaking, people think they are worth a bunch because they are old. It's not so. There is not much demand for old sewing machines. Unfortunately, there are more machines out there than there are people wanting them.
So, all that to say that it might be a good strategy to let a machine set on one of the online forums for a couple of weeks before you respond to the ad.
Also, don't forget about thrift shops, goodwill and salvation army.
We are great enablers. (I don't want to be the only one with a bunch of old sewing machines.)
bkay