One thing to consider when you decide what voltage to use... You will need a charger (one of the more expensive items IMHO) and if you start out with 48v and then go to 72 or 96v, you will need to replace the charger, or manage multiple chargers.
For unusual voltages (such as 60 or 84 volts) chargers can be difficult to find. Possible, but your choice will be limited.
__________________
|