A Raspberry Pi is not enough and a dedicated server too much? This post shows a system balancing power consumption and performance.
If you have a huge amount of servers to monitor you might use auto-updates. this is quite fine for the most of us if you have the possibility to login sometimes and do some manual maintenance-tasks. But we all know this stress during the normal day and you might not be able to do this for all the machines. And we all know Murphy so you are likely to confront the situation that your boot partition shows a usage of 100 %.