To start with your own battery pack, the easiest way is to use the 12V packs with Winston cells and BMS123. Optionally you can buy the additional cells to extend the pack to 24V (total 8 cells) or 48V (total 16 cells).
Additionally, you can use the GWL Battery pack size calculator. It will help you to pick the best configuration for your needs. The calculator provides recommendations based on your input. The desired voltage of the final pack. The required capacity of the pack. Expected Depth of Discharge during use, what % discharge cycles you will use.