The 3-phase may be a blessing in disguise. That mill has two drive systems, probably two motors, which means two VFD sets. But along with that, you get variable speed control. If the motors are 2-speed, then you'll probably want to go rotary. That'll cost a little better than $500 as a factory turn-key setup, less if you DIY. Neither solution costs more than a few hundred on top of the mill. Should be an easy pill to swallow.