The steppers and drivers you have are the same as mine, and they work well on my milling machine which is about the same size as yours. Only difference is I run them on 75v which gets you higher speed. The power supply could be better - I'd advise getting yourself a toroidal transformer for 70v if you want to get the most out of the motors. Nothing to loose by trying the one you've got first though!

I think belts are a must really. I've not tried direct drive because I think the ability to change the ratio and thus operate the motor in the best torque region is well worth the relatively small amount of money on pulleys. Also I got a heap of pulleys cheap on eBay which always helps!

When I had 1nm motors on my mill I ran them on 3.2:1 (ish) - not very fast at all. Since then I've had the 3nm motors on 1:1 which is plenty fast enough so I've not bothered testing other ratios. Different story for the router though where I used different pulleys on each axis.

I quite often manually machine with the steppers and Mach3. Either by typing Gcode or arrow keys/jogging.

Definitely Bipolar Parallel.

I used CY 1.5mm cable since SY, though available cheaper, is not as flexible. I think it has an extra layer, or thicker, insulation/shielding which makes it less suitable for constant motion.