With it being and older production machine it should be pretty ka-chunk ka-chunk with solenoids and relays. Just follow the voltage! Capacitors might be dried up. My machining center spindle drive demanded you give it power and wait at least 30 minutes if it has sat for awhile to charge caps to full 'cap'acity. Lol
I'm thinking I should go with the 20 hp. A 10hp will only start up to 5 hp, and I may get and actual vmc in the future. Most that I have looked at have a 7.5 hp spindle motor.. maybe.