Nothing really to add to the comments above and its all explained much better than I could of.:heehee:

t'll work, the only issue is that you lose z-clearance Now you've lost me,Irving? I can only think you mean when the gantry is at homing on the Z back?
He means that the clearance between the bed and the gantry will be reduced because the belt drive is in the way, so to get the same clearance the gantry will have to be raised and also the z axis will have to extend past the belts first so loosing valuable travel.

Weight won't be a problem weight till you see what I'm using for the Y axis
The weight isn't an issue as far as loading is concerned but more with moving it around, acceleration and deceleration.

With regard to the first question I personally wouldn't put the belt arrangement underneath like that purely from a damage point of view,if the machine goes out of control or you miscalculate the settings you could damage the lot, motor and lead screw! trust me it dose happen!:whistling:

I would also urge you to follow RCNC's advice on moving the orientation of the z axis rails and bearings around. a much better design IMO