A Sailrite LS walking foot does just fine on Sunbrella. I have drilled up to 10 layers of Sunbrella on my LSZ (the zig-zag model) without any trouble. Suggest you get the Monster Wheel option because it both gives you a handy crank for doing one or two stitches at a time and because it makes for serious drilling power in heavy materials. Note that zig-zag is not needed, or even desired, for canvas work (zig-zag is used on sails because sails stretch slightly and the zig-zag stitch flexes a bit so the seams don't tear out). Straight stitch Sailrites are sometimes available used at the price you are looking for (most people want the zig-zag model either for sail work or because they think they will use it on canvas). The clones are not recommended because of the lack of the Monster Wheel and because of the lack of vendor support (by the time you get a clone tricked out to where you want it you are within the price of a 4 oz cone of V-90 thread).
Note, I also have old Pfaff 130, which was considered the cat's meow in do-it-yourself sail and canvas repair in times past. However, it just doesn't have the drilling power of my Sailrite. In fact, trying to sew a blue jeans belt loop back on using my Pfaff knocked it out of time, while my LSZ make short work of it with no heavy lifting.
Note to mariners, you can store a LS/LSZ in a Pelican Model 1550 (I think it's a 1550) waterproof case if you take the motor and motor mount off the machine and store under the arm. The machine fits in the next size Pelican (1600?) with the motor unmoved, but the 1600 is lots bigger.
Note to all regarding Sunbrella, serious basting tape for Sunbrella is available from Sailrite at $7.50 for a 60 yard roll. Staples also work, though one must take care to remove ALL of them afterwards or they rust. 3M Spray 77 also works wonders for tacking together large patches and panels prior to sewing.
Excuse my while I go finish up the main sail I am working on (nearly done). In a couple of corners it has 10 layers of 7.4 oz resinated sail cloth.