Thanks for the feedback.
During the design I considered hiwin a number of times as I know they will be loads better, but to get things started at least I will stick with the SBR16 I already have.
If fully expect to upgrade to hiwin at some point, hence the x axis ball screw been fixed to a bolted in piece of tube, not welded, so it can be easily adjusted to suit the smaller rails. The y and z rails can be replaced buy just drilling some new holes in the aluminium plates and a few new spaces I can mill (probably slowly with the sbr16 rails) on the machine before I dismantle it.
This also helps cash flow a little! As you say I expect this to be a rather expensive project, and spreading out the cost a little will please the missus!

I noticed on your build log you mention you wanted to use arduino and raspberry for the controller. Did you do this in the end?