Well, I bought a book on sashiko but haven't tried it yet. There is a quilt shop a ways from me that features sashiko and the owner there told me a lot about it when I was there. I'll pass along what she told me. I haven't read much about it on line so this is just what the shop owner said. The fabric used is a little heavier than quilting fabric but I think you could use 2 layers and that would work for a practice piece. The fabric reminds me of homespun. The thread she had was more like crewel yarn..... a heavy, thick yarn that will really show up when stitched. She said that using 3-4 strands of embroidery floss would be similar. The stitches are just running stitches but they are supposed to look like grains of rice laying in a row so the stitches on the top side of your work are longer (about the size of a grain of rice) and the spaces between them are short. Using a long needle allows more stitches to be loaded but I really think any needle would work. At least try any needle before going out and buying something special.
So, you could practice on a small piece and see how your hands feel without going out and paying lots of $ for a kit, etc. I bought some of the fabric because I really liked the density of it and I'm going to try it at some point..... when I get some of the things done that I've been working on.... you know how that goes.