If all you are looking for is a straight-stitch machine for back-up, I suggest finding a good vintage machine. I use nothing but antique and vintage machines (nothing newer than 1960) and have almost no problems with them, ever. I have a small machine quilting business and I do all my custom work on treadle sewing machines--I don't even like the idea of an electric motor conking out on me!

Annie

Annie, I wish I had kept my 1968 Singer. Loved that machine! I think my reference to a back-up machine was not accurate. I'm thinking of a commercial machine to use exclusively and using my Bernina as back-up for straight stitch and whatever special things it can do for me, depending on my project. Gotta hand it to you, though. I guess that treadle never lets you down in a power outage, either.
Annie

Take a look at my machines (Studio and National Sewing Machines albums) and the work I'm doing with them (Quilts and Quilting Motifs albums.) I love my treadles, but good vintage electrics are pretty easy to find at thrift stores, garage sales and auctions.

Annie

Here is a link that might be useful: Annie's albums

A truly industrial machine is going to set you back some bucks unless you find an older machine at auction or thrift store. Don't be fooled by labels of "commercial/industrial heavy-duty machine" on some of the ebay auctions. People that know have pointed out that the machines being sold are not industrial - just work-horse vintage machines with too high a price tag.

A vintage Singer 99 or 15-91 or 301 machine would be a the types of machines I would suggest. Unless you are trying to sew thru leather or layers of denim, a commercial machine may not be necessary for your business.