Not sure how I've missed this and do agree with much of what Andy says.

#1 Closed-loop steppers are the way to go, far superior, and given the amount of work that goes into building a machine like this, it's just silly not to pay the extra.
Voltage wise it will depend on the motor but on the machines we build we use 4.5Nm and 5.5Nm closed-loop motors using AC drives with 50 or 55Vac toroidal transformer.

#2 If you use a thick enough tube the Y-axis support isn't a great loss for a wood router, but if you are wanting this machine for mostly cutting aluminum then support is better. The idea regards making it removable won't really work very well because it could affect the linear rails by pulling or pushing on the top tubes.
Now the idea of feeding through from the side is not too daft but I don't think it's worth leaving the brace out for when you could easily feed it through the other way if you just made it slightly wider.

#3 The extra plate is a good suggestion and you kind of do already have it, but I will go further and suggest you change the gantry sides and how it mounts to the ball-screws. I would shorten the gantry side to the level of the top plate and have it bolt into the top plate, this plate would also fasten into the gantry profile on the underside, this would then make the gantry a one-piece unit that bolts into the bearing plate.
The ball-nut mount drop plate would fasten into the lower bearing plate, this then makes each ball-screw and linear bearing a separate unit and much easier to align. The gantry then just bolts into the bearing plates and makes squaring the gantry much easier and without affecting the ball-screws. The way you have it now if you try to square the gantry at one side it will twist the screws unless you shim the end bearings accordingly.

#4 Check your holes.. What I mean is some of those plates won't allow you access to the bolts for the linear bearings so can't possibly work. Same on the Z-axis, make sure you can access the bolts and the bearings don't conflict with each other.

Other than these things it's looking good and your nearly at the build stage.