The helmets are allowed, but FX/AM is on it's way out and not recommended. MRCE/eFX require modification for approval. Neither are allowed for centurion (and with FX/AM also EIB) approval, but you can still get legion approval with them.
I recommend that you do not get either of them however. Start by reading this thread and you'll get a better understanding on how to get good quality without resorting to e(vil)bay.
http://www.whitearmor.net/forum/index.php?showtopic=11538