If you assume your connected machine is going to get p0wnd, and you rely on the air gap to prevent your secure machine from being penetrated, you could run any OS you like, no matter your opinion of how much the vendor cooperates with the NSA.
After all, if you're going to all this trouble and inconveniencing yourself in the name of security, what's a touch more inconvenience with using an operating system that you're less user-friendly with?
In the first case, a USB key bought at a big box store might be full of malware. In the second case, the big box store is the perfect place to buy something, as long as it's not the store where you always buy stuff, because the APT wants to keep his profile small.
You could physically destroy the wireless capability. But not using or destroying the media inputs would leave you with a fancy typewriter.