Hi Andrew:
I have a new Raptor that I recently built. I am using a Tekno ElectroSpindel C41/47-D-DB-P-ER25-1.8KW. I believe this spindle weighs 9kg. I could not even imagine trying to use a spindle almost 3x as heavy on this frame. Even with the frame bolted to a heavy table/base (mine weighing about 300lbs).
Mine does not have an ATC as you are interested... it has an ER20 setup. I am not sure what is available in this size/power with an ATC.
I believe this spindle power is perfect for the XZero frame to cut aluminum. You can see some of my videos cutting aluminum on the other threads here. The other reason I went with the Tekno is that it is air cooled, and has dual bearings. The Tekno is definitely worth the additional cost over the Chinese models.