I've built a rack and pinion and a HTD belt drive cnc before but have wanted to build a ballscrew system for a while. Especially as its small the parts should be less expencive. I'd have thought RM1605 ballscrew which seems very common would be fine given that the Y axis travel is only 700mm however RM2005 might be better or just overkill. Also do I need two ballscrews on the Y or could I use just one?

Yes your right about the backlash, it'd need to be very low ARC sec gearboxes (expencive) or 5:1 belt reduction which I use on the rack and pinion.

Good to hear you machine corian at 5k, now that is slow. I guess its a fairly high torque spindle/machine.