Chellie he said he was stubborn I think he is going to fly the plane no matter what.
From Jeffs chart on props it looks like a 11x5 would give you plenty of power with a 4 cell battery. Problem is you need to figure out the center of gravity and how much wgt. you need to get the balance. Kinda hard to say how many mah 4cell battery you will need. You have a big learning curve jumping into this hobby with that plane and converting it to electric.
Here's some good reading for you on selecting a power system.