I am building a 5' by 9' cnc router and have plans to use it on wood and some composites- My question is, what torque does the z-axis stepper have to be? It seems that is doesn't need alot of torque, as compared to the other axis.... all it does is move the router up and down...am I wrong to assume it needs less power and torque to do this ? - I am eyeing a bipolar stepper motor of 180 oz-in for the job....
HELP!

That will be fine. The weight of the router can be offset with a spring if need be. Even if it's slow, it only travels a couple of inches typically.

Eric