Bob;in your very comprehensive list, you forgot something crucial: what type of batteries to use.

This is no trivial matter. The least expensive would of course be the flooded lead-acid. It would be the heaviest pack by far, specially if one uses one of those deep-discharge batteries. A "standard" lead-acid would have a very short life as a traction battery.

That would leave you with either NiMh or Li-ions. Quite expensive, but most importantly, they require a sophisticated charging algorithm, temperature sensing and ensuring that the charge balance across the cells remains within certain tight limits.

The true smart man is the one who appears to be dumb in front of dumb man who appears to be smart.