m542 drivers are good, glad yours arrived.

3Nm motors will be good for that size. You could get away with slightly smaller motors but the cost saving is minimal so I wouldn't bother. These are currently the cheapest:

http://www.cnc4you.co.uk/index.php?r...&product_id=67

You want a PSU just short of 50V. The cheapest way is to buy a toroidal transformer (500VA minimum for four 3Nm motors, 35V max), bridge rectifier and capacitors to make your own PSU.