For clarification I am not a professional sewer. However, I have attended three weeks of sewing instruction at a marine canvas school.
First of all, I have followed several Suner posts and am totally in awe at what he and his wife have achieved – they are real talents. As for the rest of us, assuming portability and sewing sails is not important, and you are really serious about sewing several layers of canvas, webbing, glass … what you really might want to consider is a made in US, Japan or Germany industrial, compound walking-foot -- has both drop-feed (bottom-feed) and needle-feed (top-feed) with an alternating presser-foot -- sewing machine. Very nice to have features are reverse (some early machines do not), a large bobbin, knee or foot pedal foot lift and a servo motor. I think that Ccguy is correct that for other than repairing or making sails, zig-zag is not important.
The following links might be helpful, and do click on "show more" on the YouTube video.
http://en.wikipedia.org/wiki/Sewing_machine#Feed_mechanisms
http://www.youtube.com/watch?v=V-QjJgJKKVM
Sailrite is a good source for materials, notions and tools, and also for video tutorials. The Sailrite sewing machines are reported to be made in Taiwan or China and do not seem to have a good reputation with folks who sew canvas for a profession
I think that the following books are worth consideration, especially the Casey Book:
The Complete Canvasworker's Guide by Jim Grant
Canvaswork & Sail Repair by Don Casey
The Big Book of Boat Canvas by Karen S. Lipe
Beware that the number of layers you might want to sew can mount-up quickly. Sewing a simple mesh sun-screen with webbing at the corners with D-rings there were four layers of mesh and four layers of webbing, resulting in the maximum that could be gotten under a typical foot without knee lift.. The hem was only folded back once and the corners mitered. Had the hem been folded in on itself again without mitered corners, the number of layers of mesh would have been eight.
Also beware that sewing is the easy part – design and patterning, including the sequence of operations is what is really difficult.
I started with a Sailrite machine that I could not get adjusted even with excellent phone support, and returned it. A few years I tried again with Pfaff 130, often called semi-industrial and popular with some sailors, for $225 on craigslist, and that is similar in capability to the Sumner’s singer. While it sewed well, as a beginner I had little success and then acquired a circa 1950 Singer 111W155 via craigslist for $325 with none of the nice to have features plus $125 for a servo motor -- wonderful machine on which I completed several projects and that because of the lack of nice to have features I reluctantly sold and acquired a Juki 1508.