motor size?

I need some help on a lathe conversion that I started. The machine is a
14.5 by 40" manual lathe. I have a ballscrew with a 5mm pitch installed
in the cross feed. I currently plan on using a 2:1 timing belt
arrangement to couple the motor to the axis. Before I get to involved
with this I would like some input as to what size stepper motor I need?
I realize this will depend on several factors that I have not enclosed.
What I'm looking for would be successes others have had in similar
retrofits.

Thank
Brian