Stepper Size Selection

I want to convert a small-mill (large minimill) to CNC and have a
couple of very basic questions:

1. How does one initially determine the size (torque) of stepper-
motor required?

I can check the static torque, on the leadscrew, to move the table
but I imagine there's more to it and that dynamic torques are what is
really needed.

2. What are the trade-offs between using a stepper directly coupled
to the leadscrew versus one connect with, say, a stepup/down toothed
belt drive consisting of a sprocket on the motor and another on the
leadscrew?

Obviously there's a torque/speed tradeoff here but are there any
other considerations related to dynamic belt-stretch for example (or
anything else)?