I really don't suggest buying a kit, they are very overpriced. If you can find a machine shop to build an adapter plate and coupler for you, that would me much much cheaper. Some necessities are:
Controller
Batteries
Motor
Adapter Plate
Coupler
Contactor
Cables + lugs
Potentiometer
Some websites for EV parts are
www.cloudelectric.com
www.evparts.com