Chaz,

You seem to have money to spend on this which is why I am keen to see what you come up with. My suggestion on the spindle arrangement is to make the box section adjustable so that you use two of the rails as data to bring the other two in to take up the tolerances.

Getting two carriages per rail should go without saying, but you may get away with two per rail on the back and one per rail on the front.

Do you have the powersupply for the 4KW spindle?