Is it hard to built a stunt kite ?
No, it is hard to Build
a kite, once it's Built
you tend to look back and wonder why that was so hard
Do I need a special sewing machine ?
No ... all you need to get started is a basic straight stitch.
The last few kites and a 600 foot + tail we've built have been on a New Home machine we got at a garage sale for $5.00 with a bunch of books and fabric included in that price. I prefer having a zig zag and a 3 stitch zig zag but Ian's right, a basic machine will do. Check your thrift stores etc for a decent used machine.
You'll need a snap blade cutter, some decent scissors, a soldering iron or woodburner, a cutting surface, a seam ripper, something to mark fabric with (I prefer tailors chalk there are wash off pens too), and I get a lot of use out of a 36" or 48" metal ruler.
Past that some patience and research, some good sites out there with step by step instructions, read them. Check out the forum at kitebuilder.com, they are geared more to helping people build kites.