2 cells will make your life much easier. I don't have a driver to recommend that fits the stock pill. The one you mention seems to have a strong following. Hopefully, someone can give you some good info.
If you don't need modes, you could go parallel cells and direct drive (no driver). You just need to get the emitter on copper and have it well connected the light thermally. If you do direct drive, you could get rid of that crazy long driver spring and solder the positive wire directly to the switch. Then you just need to ground the led negative. Here is a pic of a good diy parallel cell holder that dchomak made for his DST (Post 562 of this thread):
EDIT: I'm saying this part more for other people that may read this post. If you go parallel, be very careful to not accidentally create a short. You sound like you already know that could be a very bad thing. You want to make sure you always put the cells in the above rig facing the same direction and to put the above rig in a insulated tube. You need a spacer anyway since the 18650's are quite loose in the DST tube.