Hi Rich, and welcome.
Almost any general-purpose power transistor these days will operate fine at 20kHz. For 600W though, you'll probably end up running multiple transistors in parallel.
Are you planning on controlling a regular 120 or 240V motor, or a low-voltage type? That will limit the choice of suitable devices a little.
MOSFETs, though pricey in high power versions, do offer a lower source-drain resistance when saturated than the equivalent emitter-collector resistance of their bipolar counterparts, so you may want to reconsider whether it's worth the extra for the improved efficiency.
There's no need for a full bridge arrangement if you're only ever going to apply power to the windings in one direction.
