I would make the mounts for the X and Y stepper motors/ballscrew fixed end bearings stronger since they have to support the axial force on the ballscrews, so any deflection in those mounts directly causes inaccuracy in the tool position.
On my machine I have SBR20 and SBR25 rails and it is clear that the 25mm rail system is substantially stronger, presumably (apart from the obvious reasons) because the bearing blocks have an extra row of balls. I urge you to consider at least using 20mm rails as it shouldn't cost that much more, although it does mean increasing the box section size to 50mm.
Have you considered using pulleys and timing belts to drive each axis? This can simplify mounting the motors, reduce resonance, improve feedrates or resolution, among other things.