I forgot to answer the motor question...

You could use less than 3Nm motors in places without reducing the performance, but 3Nm motors are by far the best price per Nm so it's hardly worth getting anything smaller, especially since the 3Nm motors would be good for the 4'x4' machine if you decide to make it. Currently the cheapest place I know of to get them is here:

Nema23 3.1Nm

Remember to ask Chai to make the end portion of the screw longer (25mm is good) to make it easier to fit and align the pulleys. This will of course mean adding a little to the length of the screws you buy.

The gantry is currently quite tall so it look like it could bend 'backwards' (parallel to X) relatively easily. It would be a good idea to add in some diagonal pieces as you said to combat that.