Wear what your passengers are wearing. If they are wearing body armour and carrying assorted ordinance get yourself some Nomex, body armour and have an assault rifle tucked away in the cockpit. If they are wearing immersion suits then do the same. If they ponce about in jeans and T shirts then a white short sleeved shirt with loads of rank braid and black trousers marks you out as the boss.
Modern turbine engine helicopters don't burn, anywhere. You have more chance of dying in an executive twin than a modern twin engined helicopter. When was the last JP1 refuelling fire? How can an instrument panel catch fire? Smoulder, yes, but burn? Engine fires? Spurious to real is about 1000 to 1. Even the real ones blow themselves out when the fuel is shut off.
Wear what you are comfortable in. Should you be a doom laden worrier then put on the whole nine yards complete with helmet. That's OK; I have had F/Os who have looked like astronauts but they are comfortable so they will fly well.
Me: I always wore the minimum I could get away with because I was always a bone idle relaxed flyer.