Off topic but very important: Have you participated in the RepRapSurvey.org yet? If so, thanks a lot! If not, please do so!

On topic:
If you have a Z-axis that runs smoothly (e.g. by using bearings for the Z-threaded rods), there's really not much need for a heavy NEMA23 motor. For the Darwin's X and Z axis, a NEMA23 motor is only heavy and expensive. This is an alternative setup for your Z-axis to run it of the cheaper NEMA17 motors instead. For using a NEMA17 or your X axis, see my other thing:
thingiverse.com/thing:1525
This will also reduce the moving mass that the Y-motor has to move back-and-forth. You can then use a much lower current for the Y-motor or also replace it with a NEMA17.

For the whole Z-axis assembly, I've now got everything uploaded. I made parts to make a pulley, skate bearing brackets to fit onto the corner brackets.