That's because vendor wants you to buy the PSU he has on his site, the recommendation is meaningless as he doesn't know intended use.

Single ballscrew on that width isn't going to work, the gantry will rack and bind if you cut towards the edges.

Inertia of 25mm screw is too high for a 2.8Nm motor, you'll be severely constrained on feed rates as you won't have the acceleration to change direction quick enough. Unless you go with fixed screws, driven ballnuts.

You may well have made a usable machine before but from your description this is in a different league and needs much more thought and design. We appreciate your supply difficulties but you're wasting your money by rushing to buy the wrong kit.