X isn't long enough to warrant a 20mm diameter screw, so use RM1610.

Gecko G540 is a waste of money - get something like this:
CNC Micro-Stepping 57 Stepper Motor Driver 2M542 2phase 4.2A Switch Controller | eBay
Or better, a higher voltage driver, like DQ860MA - cheap at AliExpress.com - Online Shopping for Electronics, Fashion, Home & Garden, Toys & Sports, Automobiles from China.

These motors are good and probably cheaper than what you specified.

25mm profile rails are completely over the top, so unless you've found some cheap ones on eBay I'd go for smaller rails - even 15mm would be fine for this size machine.

Your gantry could be made a lot stronger without using more material - flat plate is the worst cross section for the gantry since the forces are often in the direction where it is weakest. Box section would be better, or even better remove the gantry sides and have the Z-axis operating below the level of the X-rails.