My first CNC Router *PAULIC* (the machine)

[Jonathan]

Normally I'd say make the Z-axis as short as possible, but if you really want a B/C axis head then you do need a long Z-axis. What drive mechanism are you planning on using for the B/C axes? Either way the gantry needs to be *much* stronger. Look at some of the past build logs on this forum to get ideas.
You should run the motors on about 70V - drivers are around £35 each from China, which will be far superior to the TB6xxx ones.

Z is fine with RM1605, Y is best with RM1610. No point using a rotating nut on Y or Z as it's not near the critical speed of the screw.

With X you're on the borderline for using RM1610. If your maximum travel is 1350mm, then you should be able to use a 1450mm screw quite comfortably which would be fine with RM1610 without the rotating nut. If you want it to be a bit longer, then the rotating nut with RM1610 will work very well. Putting both pulleys on to get 2:1 and 1:2 is a nice idea, the problem is it doubles (ish) the moment of inertia, which will greatly affect the acceleration you get. For that reason I would stick with just the one pulley and make the shaft as short/lightweight as you can.