If the majority use is 10-20 oz duck canvasthen a walking foot is probably overkill. If you think you’ll be quickly moving up to heavier stuff like full grain leather, stitching through foam padding, etc. then it’s worth it in the long run.
Honestly for that weight of fabric you could get by with a decent home machine, the big thing is you have to use an appropriate heavy duty needle and upgrade your thread to a bonded nylon or something similar that can take the extra tension and won’t fray as it’s passing through multiple layers.
I was using a Singer 4423 Heavy Duty home machine for a while and was able to run 4 layers of duck canvas, or two layers of full grain leather pretty easily with the appropriate needle, thread and tension settings. http://www.amazon.com/4423-Extra-High-Machine-Stainless-Bedplate/dp/B003VWXZQ0