Welcome to the forum.

My experience is 600mm is the maximum width for single central ballscrew. Even then you have to be mindful of hard cuts at the ends of the travel, but in general it's fine. Above that it is double ballscrew all the way (or drive from each side anyway).

As Clive mentions single or double motors will work. Draw it out and take your pick. I like double motors as then you can buy lots of the same motor spec and driver and use them on every axis. Not a strong reason, but can make things simpler.

Something to point out is that in post #1 the last picture (5-8.jpg) shows a better machine in the top corner with double ballscrew drive. This will cut wood and plastic well, but you can go a step further than this and remove the gantry sides (weak point) and raise the X axis rails up to meet the gantry cross beam. Use a series of legs to support the raised X axis connected back down to the bed and for not much extra effort you will have a much better machine.

Good luck with the project . . .