If your machine really is 1350mm square (X & Y) then a single ballscrew is not up to it unless you are cutting foam/polystyrene or similar. I cut mainly wood on a ~700mm wide machine and upgraded to twin X.
600mm width is a rough transition point from single to twin X ballscrews.

40mm steel box section is also on the small side unless by 'frame' you mean just the bed, in which case it might be OK if you are using lots of them for crossmembers. If they are for the gantry etc then this is too small for anything other than foam or similar.
If that is all you have to hand then you could weld them back-to-back to create bigger pieces, but if you have a choice you need to go larger. Frame 60-80mm, gantry 80-160mm are better sizes, and at 1350mm machine size I would aim for the upper end of these numbers.

For a machine this size I assume you are cutting wood, acrylic, etc and not aluminium.

On the plus side the profile bearings and 2kW spindle are both good choices.