For heavy/multiple layers of fabric, you want a machine that pulls the fabric through from on top and underneath. I've been using a Juki industrial for many years now and I'm very pleased with it. I've built, modified and/or repaired a wide range of textile products over the years. Body armor, assault vests, holsters, horse blankets, dog beds, etc. I can work on just about any type of nylon fabric, cordura, ballistic nylon, rubberized nylon (Zodiac boat material), webbing, canvas, and leather.
However, those machines can jam up on you if you aren't careful. It's not a common thing but the thread can get hung up in the bobbin. Once that happens it takes a little bit of patience to re-tune your machine so everything lines up and runs smoothly. The thread is pretty substantial, so when it jams up it's like working with heavy fishing line.
And as the others have already mentioned, keep an eye on where your fingers are unless you are into graphing fabric onto/into your fingers/hands. The needles on those machines are equally substantial and will have no problem punching through. And if that were to happen, re-read the aforementioned part about having to re-tune your machine.
I would also add Rockywoods Fabrics (
http://www.rockywoods.com/). It's a family-run business in Loveland, CO. They have a slew of material and hardware for the textile designer. They started out as just another small, everyday fabric shop and are now handling large DOD orders for materials. I've known them for at least 15 years and have never had a problem with their service.