If you're building an extruder from scratch, why not build it with an auger screw, so it can handle pellets rather than just filament? Most thermoplastics come in pellet form, and are a lot cheaper that way than when extruded into filament. It would also give you more flexibility in terms of nozzle size - you can design a set of interchangeable nozzles, so you could switch between fast and thick deposition and slow and fine. If it works, let me know, because I want one...