A rechargeable receiver pack is one of the first things that you purchase as an upgrade. The next is a failsafe to prevent you from losing the car if the radio fails or glitches.
You need to see what style of pack you need first, either a flat or hump style pack. Then I would recommend purchasing as large a capacity NiMH pack of the style needed as is possible. I would suggest no less than 1600 mAH capacity.