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
Unsupported devices can always become somehow supported and enabled. To harden computer, you remove WiFi cards, cut on-board antennas, desolder mics and speakers, desolder USB ports...
I did that for computer that was used to sign bitcoin transactions offline. User typed hashes manually...
> Unsupported devices can always become somehow supported and enabled
Be very careful if anonymous developers suddenly contribute OpenBSD drivers for every single component in the 20 year old garage sale laptop you use for hosting an "online store" on TOR
Just joking, I love OpenBSD