I would look into steel tubes. I have not had a chance to test aluminum, I'm not sure How well it can hold up to the high point loads the bearings put on it.
Before you try and replicate my design, you should find a more appropriate pipe.
Nema 23's are not necessary. This uses 2 17's per axis the forces should never get high enough to need more power. nema 17's get rather large, and this runs fine on 42oz/in. If you are concerned just get the biggest 17's