The way you've orientated the X-axis linear bearings will make them difficult to align parallel - you would have to cut the frame they attach to extremely accurately or have some adjustment. If you rotate them by 90° then it's easy to align them as you can fix down one and use it to get the other parallel.

Having the Y-axis ballscrew close to the bed is a good plan as that reduces the overhang... which will reduce deflection parallel to the Y axis. Similarly to the X-axis you've not got a way to adjust the spacing in the Z direction of the Y axis bearings. That orientation is good .. you just need to have the plates in the YZ plane bolt to/overlap the plates in the XY plane. Look at how I did it on my machine if that's not clear.

Where's the X-axis ballscrew(s)?

Putting the rails on the moving part of the Z-axis is stronger than having the bearing blocks moving when it's at the upper limit. Be careful with making the Z-axis too long. It will make the machine much less rigid. I had that problem (400mm Z axis!) so I ended up eliminating the gantry sides:

http://www.mycncuk.com/forums/showth...ding.../page11
(only need to look at the last few pages...)

Mounting the stepper motors on posts has poor torsional stiffness. A 'solid' mount would be better.