Is it a three phase motor? You will need at least 3 mosfets and if you want sine wave driver, you will need six (half bridge for each phase). Please tell more about the motor.
20KHz is PWM frequency but do you also wish to have variable sine frequency of the output? What will be the range you are planning? (say 20-200 or 50-2000; some range will be needed).
There are mosfet drivers for three phase that can be used.
The parameters that you need to focus will be: (in this order)
1. Voltage; 2. Current; 3. gate drive (logic level or standard); 4. Rds(on)
As this is a rather low power (36V, 15A)- and you are not using a real high frequency- 20KHz, other parameters are important but not critical.