Is there any benefit to using coreOS when you don't need a million machines? How much work is it to start with, for example, if you have no idea what your scaling needs will be in the future?
Further, coreos tries to make you write applications in a 12-factor-y way, so when the time does come for you to use a million machines, you won't need to make huge adjustments to your deployments (just plug the container and init script into fleet and let it roll).
Our cloud-config implementation is designed so that you can use the same config to set up a new machine to match an existing one, and have it automatically join the cluster. Start with 3 machines and scale up as you need.
Learning systemd is one bump you'll have to get over, but all of the major distros will be using it, so they're is no better time to try it out.
If you think docker is a good fit for your application, this is kind of the next step.
Run from root partition A and only update on partition B, essentially via chroot. Downtime required for updates is the cost of a reboot. If, however, upon an error during boot, reboot into other partition which will be your last known good config.
Seems simple and straight-forward but is difficult for unmodified Debian-based distributions, so they've addressed this as a key feature of CoreOS.