Very nice work so far on the Helmet :D
Are you gonna to be modelling all the Mark V suit ?
If you are, maybe you could try to make it look more like Bungie's Concept art (more detailed, and a bit more bulky / well built):
Very nice work so far.
I'm not sure if you need any more reference pictures, but I found a few good pictures from Alessandro Baldasseroni's (the guy who modeled the Halo Wars Spartan) website / online portfolio.