What would I need to create a home alarm system

A little more info would be helpful for those interested. Would that not also require the Iris hub?

No, the iris keypad/sensors use zigbee 2.1 that works with smartthings, you can find instruccions in the community about how to add them to smarthings (btw, only the new ones work with ST), and you can use the keypad to change the state of SHM, they only problem for now is that the keypad doesnt run offline as of now, but that may change in the future.

Here you can find info about the keypad integration

Here you can find info about the sensors