However I beleive this thread needs to be moved to Cos-Play and L.A.R.P. Section.
The Helmet they use seems to vary, you are probably safe in assuming what you want, as its the details that make it what it is.
As for constructing parts you need, it may be up to you to figure it out, I am not sure how you would handle adding on accessories to the helmet beyond doing som actual metal working and making the additions. Maybe Make it out of Plastic? Something light.
You could use another Material if you want, their are resources you can use if you have the know how, such as Card Stock Printing, if you create a 3D image(Google Sketchup) and scale it right, their is a program to convert it to a print out for card stock, so you cut it out piece it together, use hardener and maybe coat it in Fiber Glass or an Epoxy.
Options are limitless. Find out what fits your budget.