Hopefully you won't need as much OSD as us, we back onto a creek but are just out of the flood area as per the maps so maybe we needed a bit more tank capacity. Would be good to see what Zen needs so you could compare the two. We too were worried about the gas disconnection time but it was actually a pretty smooth process. You need to know who your gas provider is and a number from your bill and you can lodge the application. Our tenants vacated on the 18th and our gas was disconnected on the 20th. It is annoyingly expensive for how little time the job takes.