Kewl! Folks here are finally starting to get around to doing the HALO Wars armour
These are just a few things that came to mind for me:
Helmet -- looks to be not too dissimilar from the EVA helmet, so you could try using that one and then just add that extra top-front portion with layered sheet styrene or something. You could also use the Pilot helmet, though personally I think the EVA more closely resembles the style of helmet you're going for.
Oxide Tank -- like Spawn Camper suggested, you could try finding an appropriate-sized plastic drum. One other option would be to take one of those thick, industrial cardboard tubes used in cementing patio deck foundations (doubt there'd be much of a difference in terms of weight between the two). You could cut out pieces of MDF board to seal up the ends, then bondo and detail as desired.
For the brackets that attach the tank to the shoulders, slightly trimmed-down 1" by 2" wooden beams would probably work. From what I can tell, the brackets angle down toward the shoulder mid-way; could be looking at the first part of the bracket (what actually attaches to the harness) being about 6" long, with the second part (what actually attaches to the tank) being about 9-10" long.
An idea you could employ for the tank would be to incorporate some sort of catch-release system. Have the part of the bracket that attaches to the tank extend all the way through, then have a catch mechanism of some sort at the rear to hold it in place while wearing the armour. When taking it off, you release the catch mechanism to slide the tank off. Would make it a lot easier to get in and out of the suit, imho.
The harness connecting to the brackets could be comprised of double-layered MDF board or some sort of resin/fiberglass-reinforced paper frame, with the harness screwed into the brackets from underneath (don't want/need anyone getting poked by the pointy end of a screw). Though there are no pep files yet available for this, it's entirely possible you could design your own paper model from scratch, which would work for the resin/fiberglass-reinforced option mentioned above.
Regarding the top-rear protrusion on the tank, you could take a short length of PVC piping (possibly 3" diameter) sealed on the top end, cut a hole out of the tank and secure it in place.
Just throwing some ideas out there for ya, dude
P.S.: I'd love to help out with designing the suit (though it's not my project I've already gots all sorts of ideas in mind for it, lol), if only I weren't so busy with other things. Good luck with this.