Are you talking about getting the HiTorque mill, and converting it, or purchasing one of their pre-assembled CNC mills?
If the first, those mills are a variant of the Sieg X2 (actually, the SX2). Lots of people convert them, and you can even buy kits. I recommend checking out
hossmachine.info. Hoss has taken these little machines farther than anyone else I have seen. I think he still sells conversion plans too.
As for hardware, you can get driver/motor kits on E-bay, or through vendors like
http://kelinginc.com (I got my servos and drivers from them). For software, people usually go with either Mach3, which runs on windows. Or LinuxCNC/EMC2, which has it's own built in operating system (Linux). I went with LinuxCNC, because I prefer Linux, if you are not familiar with Linux, or you are more comfortable with Windows in general, Mach3 is probably a better choice for you. Both are very capable, and for what most hobbyists do, there is not a significant difference. I hear that Mach3 is easier to get up and running, but I don't have any direct experience with it. You could try out both, LinuxCNC is a free download, and Mach3 has a demo version. The hardware (if you are using a simple parallel port breakout board) will be the same for either.
If you are going with one of their pre-assembed CNC mills, I think everything is included.