I just hooked one of the offshore VFD's 220 vac 1 phase in to 22ovac 3 phase out to my buddy's lathe. It was a larger unit (7.5 kw) as his lathe is 10 hp. This unit cost about 500 CAD to bring it to Canada off of Ebay. I was impressed with the low rpm torque. As for a motor you should be able to find a new standard duty 3 phase motor for a couple of hundred bucks in the US, and a used one for $50. Check somewhere like Automation Direct for a new motor. With your mill being belt drive it should be easy to configure a new motor mount if you can't use your existing. PS a three phase motor should be somewhat smaller than a 1 phase counter part.