What's needed for a 3d printer build??

[Neale]

I built a 3D printer a couple of years ago, before I built the CNC router. I'd got the bug...
Positioning accuracy doesn't have to be that great to get usable results although as with all such things, better accuracy and machine stiffness shows up in finished surface quality. Hot end is fairly easy to make if you have access to fairly simple machining facilities, or you can just buy one, but you will need an extruder to drive the filament through the hot end. There are a number of standard designs, most intended to be printed themselves.
I wonder if you will be able to get away with your current controller, though. One key aspect of 3D fused-filament printing is that you need to very accurately control filament extrusion rate with XY movement and I don't think that that's something Mach3 or LinuxCNC will know how to do. Most people use dedicated controller hardware based on the Arduino such as the Sanguinololu and running software like Sprinter. There's lots more to be discovered at reprap.org, and the forums for specific questions.
Probably easiest to use the CNC router to build one of the later design printers, tempting though "just swap spindle for extruder" might be!