I think part of the problem is I don't have the motor performance curve, I have worked out the ballscrew critical speed is about 1000RPM (end fixity type C - right?) but not sure what speed the motor should be going.

I'd definitely be interested in motors, drives & controllers - I'll PM you. I won't be ready for them for a few months yet so no major hurry on my part, I just need to know what size to design brackets to at the moment.