Get a motorless grinder (Like a Coote), a good 1HP to 1.5HP motor, get a three step pulley, you have a VS grinder. It only takes seconds to change the speed ( Please, not while running).
The other choice is a 1HP 3Ph motor and a VFD (plus the Coote). That will cost about $300 if you shop around a bit. California has lots of these drives and motors for sale in the industrial surplus shops.