Using Ring Alarm without the motion detector element?

We would be interested in the Contact Sensor element of the alarm system, but wouldn't necessarily need the Motion Detectors - if we cover the downstairs entry points this feels like enough for what we would use it for, personally. Is it possible to purchase the items individually, rather than as a kit, so we don't have the Motion Detector element of the kit just lying around? We would need the contact sensors, base station, keypad, and range extender... Are we missing anything? Thank you!