Don't forget the most important security considerations: (1) choose a hardware and OS combination where none of your I/O hardware (video, audio, wifi, etc.) is supported, so that it can never be used to exploit your system; (2) choose an OS so obscure and weird that a potential hacker is guaranteed to have never even heard of it, and would need to study your specific machine for months to make heads or tails of it
Just joking, I love OpenBSD