This is the kit we are using at start-up.
2 x Rechargeable 18650 Batteries with charger
These will be needed to power the car. You may already have a charger. $19.95
2 x $16.95 =A$33.90 Jaycar You may want to consider another two as spares. Charged ready to go.
Various deals for batteries and charger can be found online. You can charge the batteries in the robot.