Most all bikes even the cheepos at Wallie World will handle 250 to 300 pounds. You should be considering the total weight of the rider, batteries, drive motor, controller and any other thing he may want to carry. If you use a hub motor most will tell you it should not be considered as part of the total bike weight (for capasity). Depending on the type of riding your dad wants to do will also effect whay motor and batteries you use. Remember, lead batteries weight a ton so I would avoid them. Lipo, is not a great idea for a newbie. So I would recommend lifepo4. I would consider something like a Ping lifepo4 pack. Of course the size of the pack will depend on the motor you choose. Good luck and enjoy the build with your dad. BTW, maybe consider building 2 ebikes (one for him and one for you).