If you have a good system (either in the helicopter, or in the headset), it should give you the option to modify the sensitivity of the "hot mike"-that should take care of the ambient noise issue.
I personally do not believe in the benefits of an ANR.
To me, passive noise reduction is the way forward.
I have been using custom molded ear plugs from the beginning under my helmets and headsets.
25 years and over 8000 hours later, my hearing is still excellent...