On the topic of backlash:
For the Z axis, I try to minimize backlash by putting thread clamps and thrust bearings on each side of the lead-screw.
This seems a standard way to deal with the problem and, even with the plastic parts, appears to be reasonably effective and rigid. The photo shows that the ability of 3D printed plastic to absorb the grease that gets on your hand from the socket cap screws is impressive. Once I get to final versions of the parts, coloring might be an option. Ponoko has some information about using RIT dye on the plastic
http://support.ponoko.com/entries/20066883-rit-fabric-dye-on-durable-plastic. The picture also shows that this is not the final version of the Z bearing holder. I had to use a burr and rotary tool to deal with not leaving enough space for the thread clamp. The plastic printed parts are surprisingly strong. It was not trivial to cut through the base.