I've always been told that vacuum tube gear is far more resistant to EMP than anything transistorized. The R390A is a legendary receiver regardless, a mechanical engineering marvel with a noise floor close to the galactic level. But yeah, I heard they kept stockpiles of them til the 80s specifically because they were EMP resistant.
I only know the basics when it comes to encryption - it's best to have a radio with an ECM, which is hardware encryption as opposed to software. That's a rabbit hole, no doubt.
As far as evading RDF, it depends on the situation. Obviously, use the lowest power level possible, keep transmissions short, and when you're static, set up a hot loop/field phone so RDF becomes irrelevant. There's a whole aspect I know almost nothing about that you might be interested in - mesh networks, which are supposed to offer advantages of security over traditional RF simplex/repeaters.