[Christ]

If it's 24V, you wire the batteries in series. Sit them side by side, run a wire from the + on one to the - on the other, then Use your open + and - for the remaining connections. It's the same thing as a 24V battery, except using 2 cells.

You're probably best off gearing it at or close to whatever your top pedaling speed is going to be for the area you want to use it, with a freewheel sprocket on it. Overheating is only going to be a problem if you overload it. Don't try to take off with it, and if you notice that you're really slowing down, and you can't even pedal to keep up to speed, it's time to let off the button. You'll get a feel for it. Later on, you may decide to put the 450 on there, or you may decide that neither is necessary.

Personally, I'd use the 450 for something less utility, more fun.

Keep in mind, that if you're going to be going up the hill with this setup, you'll want to gear it so that it can pull you up the hill, but you can work with it to help out.