I would just use Linux Mint with some good passwords. You're already being very safe using Linux on a separate mostly offline device.
Mint is considered the best Linux desktop environment by very many people and is ranked #1 at distrowatch.

Basically if you fuck something up you can probably figure out how to fix it in Mint without using the terminal, other Linux distributions are less friendly.
Mint also supports HD encryption out of the box with 0 issues or hassle.

There are also some tips for hardening any Linux such as blocking ports you know you don't need, google "Linux hardening".

I've sent you a little PM. I may not be a big name here in Bitcointalk but I'm definitely an investor.