BLDC motor, typical maximum isolation voltage in DC
Motors for industrial use that conform to common international standards should be capable of withstanding peak transient voltages of up to 1000 volts. That would be for motors with nominal RMS voltage ratings of 100 to 500 volts. A 400 volt PWM voltage would probably not produce transient voltages over 1000 volts. Motors for industrial use should have specifications or markings on their rating plates that indicate what standards they conform to. Motors for which that information is not available would not commonly be considered as suitable for industrial use.
The use of a PWM frequency lower than 2 kHz is likely to result in unacceptable harmonic losses in the motor. At 2 kHz, there would likely be a noticeable reduction of motor efficiency, but it might be acceptable.