Thanks Jazz,

Regards ballscrews then I wouldn't use 16mm diameter for the long axis because in my experience it's just a bit too long. Now you can get 10mm pitch in 20mm Diameter then your much better using them over 16mm at this length. The difference in performance is neglible and your far less likely to get whip so often can go faster than 16mm screws that whip.!Use 16mm Dia 5mm pitch for Z axis.
Also I recommend you connect steppers to screw with timing belts and pulleys to help reduce resonance and add some flexabilty with ratios if required.
i was going to use 25mm ball screws for all of them, but I see it would be better with 16mm on the Z... Pulleys idea - I hadn't thought of...

Regarding the gantry sides, I am using 20mm alu as the bracket, I thought this would be plenty rigid enough based on other designs I saw - how would you size it different?

the location of the ball screw - I accept your opinion on this... My thought was because at the max depth I'll be cutting just PU foam, and this is far below the ballscrew it should be ok, and in a simple location build-wise...

Where would you try set it otherwise?

thank you for the heads up on the motors from Zapp

i have heard that Mach 3 can be unreliable - what's the consensus experience with it?

anyway, regarding the hardwear, I'm definitely going for profile rails, circa 25mm, with ballscrews on X and Y of 25mm, and 16 on Z, each with 10/10/5mm pitch respectively.

motors - definitely digital sounds better... Power likely as advised