Good choice on the stepper drives. Ive got the 542's and their really good.

I take it thats not the actual drawing of your machine but just in case, Don't mount the main y beam like that cus it will move all over the place. Extend the side supports up and fix the beam top and bottom. if your using the plate and skate bearings then also extend the plate to the side support. that will make it much more rigid.

Theres quite a few other bits that could do with changing but I dont want to be to negative