Fixed z axis to gantry design

[Lachlan]

Hello everyone,
This is my first post here but i have been a long time reader of this forum and the likes of Jazzcnc and Jason and many, many others have inspired me to take the plunge and build a simple cnc design. I have limited hand tools plus a dropsaw for this build so simplicity is a major factor. I don't have a whole lot of experience working with steel but I can weld.

I will be milling aluminium so the machine needs to be rigid. i will be building it out of 75mm square box steel 5mm thick.

After perusing many designs i think (not carved in stone) I will be basing my design on the following machine.

http://www.youtube.com/watch?v=4Bj6er322Dg
With the following alterations:
rail size will be 900x600by300
Gantry will not be fixed but will move on 20mm Sbr linear rail as will all other axes. The cutting area will be too small if its fixed and i realise this will impact on rigidity. I know profile rail is preferred but its just out of my budget.
Will be using pulleys with HTD5 15mm belts on all axes.
Nema23 425oz.in motors on each axis driving 1605 ballscrew thread.
Reasons i chose this design:
Simple to build
Rigid z axis that i know I can build accurately.
Seems like a very strong design.

So any major design faults with this design?
Am I on the right track?
What would you change about this design to improve upon it?
Lastly, thanks so much for any help given.
Regards,
Lachlan