I've built this machine 100 different times in my head!!!

[Jonathan]

Yep going for Hiwin, as I can't find any suppliers of new THK or NSK. I'm using 20mm profile rails and the carriages have 4 rows of balls.

15mm rails will be plenty strong enough. The load ratings of 15mm are several times what the machine will require... considering you were happy with round rails just a few days ago, I don't think there's much point using bigger than 15mm since the difference in stiffness between 15mm and 20mm profile rails is tiny compared to the difference between profile rails and round rails. Many much larger industrial machines use 15mm rails.

I've redrawn the motor mount, but decided not to go with a full plate on the back of the z axis. Just extra weight for the gantry to carry. Can be easily added later if needed.

If you do add one it needn't be thick - 10mm would be fine as it's only to stop the joint between the plates that the Y and Z bearing blocks mount on bending.