I had a machine similar to the 15-91. It was my Granny's machine from the sweatshops of NYC. It worked real well for a 60 YO simple machine, but a flood ruined it.
I recently bought a new Janome HD-3000. It is mostly metal and works pretty good. I was able to sew 6 layers of marine vinyl, but to be fair the Singer was a bit more powerful.
My only complaint is that the foot on the Janome doesn't rise as high as on the Singer. However, the Janome has every stitch you would ever need, and if I had some talent in sewing, I could make it sing and dance.
A leather needle is recommended for vinyl. I also believe you can get a walking foot for the Janome, which is recommended for upholstery and you probably cannot get for the singer.
JMO