A number of people have retrofitted similar hurco's with the same 90 volt motors using the V200s .They handle those motors easily.
Post some pictures of the mill and motors if you can.
You need a breakout board. PC4cnc has a bunch to choose from. You don't need a opto isolated one since the Vipers have a opto step/dir input.
You can put a VFD on the spindle if its 3 phase and leave the pulleys in the middle (1-1). but you still have to change ranges by hand.