Chris,
You don't need much power usually under 200oz will be plenty, the machine only moves at about 3.5mm a second unless you really kick the heat up but then you will burn out any square corners. If your only building a machine to built 1 plane I would look into having someone cut it for you and save hundreds of dollars and many hours. I have a 4' x 4' x 30" machine, it uses air cylinders to tension the 60" wire. (see Pics) If I can be any help let me know.
Mark
"Stupid machines - they do exactly what you tell them!"