One of the things about commercial machines, is that they do one or two things very well and very fast. They often use purpose designed feet to help in the production speed and efficiency. The good thing about that is that commercial feet are often cheap to buy. $5-$10 is rather common. They position the fabric being sewn in the proper relationship with the needle and pressure feet to allow for a uniform stitch placement. I think you might have to develop some different handling techniques to gain the full advantage to sewing with these machines at a fast speed. I remember the first time I used a Juki 5500 straight stitch machine. I stepped on the peddle and that machine took off at 5500 stitches per minute. It was like a sewing machine with a turbocharger. There are a number of machines that are used for specific types of sewing and having a talk with the people at the above referenced website, and explaining what you want to do with the machines, and how much money you want to spend, with give you a good idea of whether this is something you want to jump into with both feet, or maybe just tippy toe in a little bit at a time. The other thing to think of is that a lot of commercial sewing operations are closing down in the US, and used machines are a dime a dozen right now. That being said, they might not be a good deal unless you feel comfortable doing some maintenance work yourself. Good luck, and let us know what you think you will do. I have often toyed with going the commercial route myself, but I am having too much fun just playing around the edges.
John