I have exactly the same machine and have now used it quite a bit for 3 years. Sewing kites is one of the least demanding things (for the machine) you can. The fabric is very thin and pretty easy to sew. The "Heavy Duty" name is probably overstated though - my machine can't make it through the seam when hemming a pair of blue jeans for example. But if kite making is the main use of the machine you don't need much. I've only ever used 3 stitches - straight, zig-zag and triple zig-zag. Probably every machine made now has many more stitches than that. So you don't have to spend a lot. A walking foot is probably a necessity, and for NPW's I would get a cording foot. This is because often you use kite line as a seam reinforcement (see Tom White's plans on kiteplans.org). This provides a very strong attachment for bridles. It is very hard to sew a a reinforcement line without one, but quite easy with one.