I've been using the walking-foot unit that Harbor Freight sells for a few years now, and I've been very happy with it. It's a Yamata FY 5318, which is a knock-off of a Brother 797. It's a strait-stitch, (not zig-zag), but It'll sew through anything up to 3/8" including actual fire hose. I got it on sale for about $500 with the stand and a clutch motor. The parts are all generic industrial stuff.
A friend has the Sailrite Ultrafeed, and I'm not favorably impressed by it. The machining and tolerances didn't seem up to par, especially for the price. It just didn't sound like it was running as "smooth as sewing machine". We had a lot of trouble with the Tenara thread on his, and none on mine.
You will probably find that your old machine is good enough for most canvas work, especially for the small pieces. If it can handle v69 thread and a 16 needle, you should be good-to-go. If an 18 needle and v92, even better.
For large pieces, friends feeding and guiding the fabric will be very helpful, as the non-walking-foot machine does not grab and control the fabric as well.
I just completed a 8' X 32' awning for our yacht club, and it took 5 people to make it. Two on the in-feed roll, one sewing, and two on the out-feed roll. While this could have been done on a heavy home machine, I bet the stitching wouldn't have been as strait.