I’m actually in the process of rebuilding the https://roots.io server.
Yes you should ideally use Ubuntu 16.04 because it’s required to get HTTP2 working.
Keep in mind you don’t actually need any down time. We just spin up another droplet and use our local hosts file to fake DNS. Then once the server is ready, you switch the DNS over to the new server.
You could also take advantage of DO’s floating IP feature to just point it to a new server (I think, never used it).