As I’ve built several 3D printers, I’ll chip in.

You’ll need a few specific things.

·         Fourth Axis to control the stepper that will drive the extruder

·         Extruder – the mechanism that feeds filament at a specific rate to the hotend.  Search thingiverse.com for ‘extruder’ to get a good idea of what’s going on

·         Hotend, this is the item that filament is fed through which melts at the end.  The gold standard is here: http://hotends.com/.

·         Temperature control – this is usually built into the electronics for any reprap (Arduino mega 2650 + Ramps 1.4 being pretty popular), but those electronics also drive the steppers.  So, you’ll probably want to go with a simple analog/manual PID temperature control device.

·         Software – to take the 3D file (.stl usually) and slice it up into g-code.  I use Slic3r from http://slic3r.org/.

http://forums.reprap.org/ is a good place to ask questions.

Luck!

Jeremy