Be warned that Kelly controllers have a patchy history of ever reaching there advertised power values.
Have a look over on DIY electric car; link:
DIY Electric Car Forums - Electric Vehicle Build and Conversion Community For threads on this.
Basically go for the biggest one you can get. I cant really help with performance as i have only used a curtis connected to 96V set up in a SAE race car. We had a kelly but it was terrible even on the bench during testing.