Battery backup is always recommended.

Beyond that, what accessories you need depends upon what you will be using it for. 

If you are using the AGM to emulate a dial tone for external equipment, and you WILL NOT be using it to generate alarm messages, then you really only need the AGM and the battery.

If you are using the AGM to generate text message alarms, either for power failure (built in) or external sensors, then you need the AGM, battery, and programming cable.

If you are using the AGM to generate VOICE PHONE CALL alarms, either for power failure (built in) or external sensors, then you need the AGM, battery, programming cable, and voice module.