What to buy when ordering the C2:
A case is needed as it will not fit directly into a Pi case.
You will need to cut the official C2 case to gain access to the micro SD slot so dont forget to do this before mounting and screwing it all together.
https://www.hardkernel.com/shop/odroid-c2-c1-case-clear/
RTC shield is only $4.50 which the price includes the battery. If your power fails stopping your internet connection, then it is possible openhab when it reboots may run rules with the wrong date/time. NTP does not solve all situations so this addon is worth getting and my script will auto setup this shield for you if you edit the prefferences.
https://www.hardkernel.com/shop/rtc-shield/
Whilst you can use a micro USB power supply to the OTG port, I recommend you run the C2 from the barrel plug as it draws 0.5 watt less power when at idle when doing this. Even if you plan to re-use a PI power supply, consider this cable as a backup as the cost is low.
https://www.hardkernel.com/shop/usb-dc-plug-cable-2-5x0-8mm/
Lastly this UPS addon is a good idea but personally I prefer to spend my money on a bigger UPS that can then power other gear as well and provide surge protection. A larger UPS is at least 5 times as much to purchase as this one but will provide a lot more features and power and is also a lot heavier and physically larger. A UPS is a must for any small computer that runs on flash based storage.
https://www.hardkernel.com/shop/ups3/