Its quite a challenge. Maybe finding a bargain second-hand VFD would be easier?
Perhaps a forum on electric vehicle drives would be a more fruitful place to ask around - some
people use AC motors for home-brew electric vehicles, 6kW is in that sort of area...
I am guessing a 3-phase PWMable IGBT bridge is the minimal starting point for the
power stage, but you also need some quite sophisticated sensors and control algorithms...