Power at a speed on level ground is determined by your frontal area, wind resistance, rather than your weight.
Power up a hill is determined by your weight. How fast do you want to go up a (for instance) 1:10 hill? With a body weight of 80 kg, and perhaps 20 kg for the scooter, that's 1 kN / 10 = 100 N force. To do that at 20 km/h = 5.5 m/s would need 550 watts.
How steep a hill do you want to climb at any speed? To climb a 1:5 would require a force of 200 N. This can be converted to torque on the motor by multiplying by the radius of the wheels. Maybe you don't need to climb a hill that steep, but would simply hop off and walk if necessary?
Once you have the wheel radius, you can convert speed to rpm, or rad/s. When you choose your battery voltage, this will give you the required KVs.
Or just pick a motor in the low hundreds of watts range, and analyse what performance it will give you. It's often much easier to do several analyses of specific configurations, than try to synthesise a good configuration, especially if your criteria are not well thought out.
