Looking good. with regard to the rails being on their side with only one contact then in dosnt matter as in reallity you are only supporting the weight of the y and z axis. The plunge of the z axis will counter some this and you will then have max strength (4 contacts) in the sideways movement of the x axis where you need it to counter the cutting forces.

Not sure it I would have moved the y axis rails as well tho. For that long travel on the z you need them as far appart as poss to reduce the moment, the other side effect is that the y rail has moved the spindle futher way! You might want to shift the y axis rail back a bit further to compensate, dosnt mater if the motor overhangs a bit. mind it would be easy the remake the side plates later if it dose cause any problems. (Chatter, binding etc.)

Did you find RS the cheapest for the aluminium sections?