that helmet, as already mentioned, be hard to sight your rifle. a HUD system will probably be x20 your current invest and make operational. it would be incredibly hot to wear in any weather and will no doubt fog up your goggles. lastly, being a fully sealed unit, you'll have issues hear ambient sounds making tracking people shooting you tough in a milsim situation.
you'd also want to clear with the field operator that they will allow the use of it. what kind of lens or goggle system are you going to use with it? it would have to be ansi rated to withstand point blank shots.