I think you need to decide between belt drive and ballscrews.The ballscrews will effectively multiply the torque of the steppers and belt drive can only reduce it as you increase the diameter of the toothed wheel.I would also expect better accuracy with ballscrews,but with 1800 mm,there is a chance of them whipping at high feed rates.Perhaps if you can organise the use of rotating ballnuts and fixed screws,it will be the best of both worlds with good accuracy and abundant torque from a modestly sized NEMA 23.