Good start - almost getting jealous!

A few general observations...

Have you seen the BT30 ATC spindles on Aliexpress? If you're happy with 6000rpm then they're not excessively expensive:
http://www.aliexpress.com/wholesale?...initiative_id=

PSU - why no decision? A toroidal transformer of around 500VA would be ideal.

What is the mass of the Z-axis (head, spindle etc) and what are the dimensions of the Z-axis ballscrew? I've made a script to calculate it more accurately, so if you post that information we can make sure the motor you've chosen is the best option. I suspect you may be OK with one of the 3-4Nm Nema 24 motors. The gas strut makes surprisingly little difference if you try to get a reasonable acceleration...

Correct me if I'm wrong, but it looks like you're leaving the back of the Z column open, so it's a C-shape section not a box? If so then you should defiantly put another plate on the back, as the torsional stiffness of a C-section is very low compared to a box. That's one of the big advantages of putting the ballscrew on the front of the column, not inside - you can have a closed section.