Looks good, i HIGHLY recommend the G540, and not to put any bad words to kelling, but Ahren over at CNCrouterparts.com, has the plug and play set up for the G540 and the motors, and let me tell you that is so nice to just plug everything in, NO SOLDERING!
My machine has worked flawlessly and i am in the process now of going to a 4th axis, and building a second machine that will do 4 x 8 foot sheets.
Second not on the twin ball screws, best way is to run a belt and pulley system, just probably have to gear it a little so the motor has a little more tourque, which you would lose a little speed but its ok. I have had great luck with B and B Manufacturing (http://www.bbman.com/) great service and help, fast shipping and the products are so nice.
Let me know if you have any questions, just PM me