As irving has said, you need to use 10mm pitch (RM1610) ballscrews on the X and Y axis to get respectable feedrates without using a larger diameter ballscrew.

Clearly it depends on what the minimum feedrate you can accept is, but I wouldn't go above a 1500mm long RM1610 ballscrew without using a larger diameter screw or rotating the nut. If you're interested I can make the rotating ballnut mounts for you, let me know if you wanted the details. I wont be able to make them any time soon, but it looks like it will be a while until you're ready anyway.

The 3Nm stepper motor shafts are 8mm, so you would need 8mm to 10mm couplers. However I would strongly advise using timing belts and pulleys since this enables adjusting the drive ratio to get better speed or resolution with the added bonus of adding damping to the system to suppress resonance.