My suggestion would be to use the 540 for X,Y,Z, and rotary. If you are only indexing with rotary then when not in use you can
use it for another axis or as a spare should one of the other axis drives go bad. I manualy crank the knee for rough setup and have the quill movement done via a ball screw setup. You need to plan for the future ( I don't know what your up to......)
and if you are going to be usng 6 axis then the power supply needs to be adequate for all of them.
Since the G540 is limited in axis amps you may want to consider one of the smaller Gecko drives and save some money for just a single axis addition.
Back on the quill......Some folks have counter balanced the complete mill head for movement and can be done with a small stepper
where it was easier for me to just add a ball screw to control the quill, but if deisred i can disconnect the screw and manualy
use the quill ( if i will ever do that .....). The Z accuracy is important and no backlash is very desirable so you have good repeatability. The 3D
work will require it and make sure the axis have adequate torque for good acceleration.
FWIW,
RICH