Voltage
4S is often simpler for beginners; 6S can be efficient but needs compatible parts.
FPV batteries
The battery affects thrust, temperature, weight and flight time. Choose it together with motor KV, ESC voltage limits and the rest of the electronics.
Check a battery in the builder4S is often simpler for beginners; 6S can be efficient but needs compatible parts.
More mAh also means more weight, so bigger is not always better.
Look for enough current delivery for the motors and props you choose.