[Christ]

Water pump -

Depends on the vehicle as to whether you have to design it in to your plans.

Most cars with timing BELTS have the water pump driven by the belt, so you don't have to compensate for it at all. (It's still a power drag, though, and there are WP delete kits that relocate your Tbelt and run the pump w/ an e-motor for some cars. Honda comes to mind.)

If you already have an electric water pump, you can get an electric one, a kit to make it electric, or just leave it belt driven. It's one of those things that you don't want to do without, for sure.

If you have a fear or running out of charge, you can always use a starter/generator motor to do what you're suggesting, so that you can just reverse the field and it will charge instead of drive, but you'll still have to have a way to mechanically link it to the crankshaft of the engine. (Like another belt.)