My recommendation? a lightly used Pfaff mechanical machine is what to seek-out, USED
it has a "walking foot" which grabs the fabric like the jaws of a pliers, from both the top and bottom. That makes a huge difference when you finally begin working with super slippery material like Icarex and a seven foot seam!
I've seen kites made with just a straight stitch (as opposed to the triple stitch zig-zag, as that was all that was available on that old model, almost like a manual with the foot pumping mechanism.
After my first kite sewing retreat struggle, I gave that machine away, promised the bride we'd never own a kite I couldn't acquire w/VisaCard, but eventually a bunch of great kite builders in my local club forced me to learn how to do it myself. (particularly Dave Ashworth, he only flies stuff he built himself, never the same twice!)
That Pfaff led to an even greater appreciation for the workmanship of a true master builder, such as Shook or Bazzer. I took lessons from a guy who owned 7 different brands, each time I had to learn a new one, you learned what features were most desirable and then I found machine that could provide those desired functions as a reasonable cost. Low bottom bobbin warning, needle down/stop locking, walking foot, a fitted table so more of the fabric is flat going thru the machine. Lettering and advanced (Kewl) stitching features are a computer as opposed to a mechanical feature, costs will also increase accordingly.
no one will EVER care as much as you yourself do when building YOUR kite, but that doesn't mean you can't get something made as a custom, perfect and your way, for a little bit of extra cost, try ASKING for it.
The best thing about doing it yourself is the sense of satisfaction when folks converse about your creation, or enjoy flying it themselves. Changing something for testing purposes?,.... what happens if? Being a builder make these considerations fun to explore, most resulting in failure, but occasionally you hit a home run with the bases loaded. These are magical moments to savor or share.
Being a builder allows you to work cooperatively too. From my experience this is the single most rewarding activity in kiting,... your entertainment choices may vary
-plm