I'm just over your weight and my 36v 450w geared hub conversion does about 17mph/27kph (level ground, no wind, no peddling).

My 36v 750w direct drive conversion does about 20mph/32kph.

If you're going to "error" in the estimated requirements of your power system I'd do so on the plus side; i.e. go with a 48v battery, a 15-20ah lifepo4 battery, and no less than a 750-1000w hubmotor (which is what you seem to be thinking as well).

The key here is that you select a well balanced power system (as possibly mentioned above). Short changing anyone of the three above "ingredients" (system voltage, battery ah, and output wattage) will, most likely, leave you "wanting" in speed, hill climbing torque, and/or range.