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

[Jonathan]

Definatly use the 'heavy' extrusion if you can afford it. What profile rails are those and what size...hiwin?

A screw will go from the bk side, through the z rear plate and screw into the motor mount. Hope that is strong enough.

It probably would be strong enough, but it would be good to have a second support for the motor mount, like this one I made earlier:



Much more important though is you need some method to tension the belt. The easiest way is to instead of having simple holes in the motor mount, make them slots so you can pull the motor/belt tight then tighten the bolts - again like in the one above. This also allows different pulley sizes to be used without changing the belt, which could come in useful.

Edit: Just to add, moved sketchup discussion here as requested.