Will those Y motors be running in parallel from a single driver, or on separate drivers with separate limit switches for homing? I had imagined them being mechanically linked by belt, although that isn’t without its drawbacks either since it has to be readjusted to get the axes perfectly perpendicular every time you remove the belt for maintenance. But it does allow moving the axis by hand.
A thought: See if you can move the X motor up on top, above the 4040 extrusion so the Z unit can pass in front of it. Might give you a little more usable X range.
BTW, these are the NEMA23 motors I used. Very cheap and good NEMA 23 Stepper Motor - CNC Mill Robot Lathe RepRap Makerbot 3D Printer P13V | eBay
Mist coolant certainly would be good for running inside a closed box. Mine requires constant attention for most tasks, applying coolant ahead of the cutter and clearing away chips. Especially for 1/8" and smaller mills. 1/4" doesn’t seem to care much about coolant, except that sometimes it gets aluminum galled onto the cutting edge. I think zirconium nitride coating is supposed to help with that, but I haven’t tried it.