I have a Singer from like 1975 and it works great. Bought it used, and cheap ($60).
You need the 3-step zig-zag stitch. Without that, you really can't repair canopy as the pros do, you'd be stuck with a regular zig-zag, which I've never seen used on canopies. Of course, you do need the regular zig-zag for your line sleeving.
For some reason, the 3-step zig-zag stitch is not so common in the older machines I've looked at (1970-1980's). Beware, some machines can do it, but require a cam for that stitch, and if the cam doesn't come with the machine, well, start searching on ebay...
Pay attention to the max width of the zig-zag. 5 mm should be enough. Should always be fully adjustable (the width). Measure the widest zig-zag stitches on your kite to get an idea.
Most if not all consumer grade machines have a hard time with a leading edge. That is usually the thicker dacron which is often folded over, making it difficult for a regular machine. It can be done, but might look crappy if you have to stop and start a lot because the machine is struggling.
The length of the arm is important, especially when you have to get to the center of the kite. Some of the smaller machines out there have a short arm. Mine is medium at 6.75 inches and I've gotten to the center of the canopy on a 16 m kite.
As I recall, there was another thread in here about sewing machines...