first, you have to decide on what batteries... if you go with the less expensive floodies you will also get better range and durability, but more weight and space for given pack voltage. zivan, elcon, and others are not too expensive.
I would suggest you strongly consider going Lithium rather than adding another set of lead, especially if you have to buy a charger anyway. If you work the numbers out over the life, the lithium is less expensive per mile of service.... and will perform way better at a given voltage pack being so much lighter in the car and with almost no voltage sag. Just need to change your instrumentation a little to count amp hours instead of voltage based SOC meter.