It's the torque that is ultimately going to help you wheelie. Power is a function of torque and RPM. It sounds like max RPM doesn't matter, so go with a motor with good torque, and gear it so you have gobs of torque at the lower end. Motors produce all of their torque at 0RPM until a knee (sometimes 2-3kRPM). You want to wheelie? Gear it so all of that RPM range is where you want to wheelie.