sounds like it is all coming together.
I was actually thinking today on how a hev suit would go together, but going without Gordon would likely be a lot better. (he wouldn't be able to have dialogue, so what's the point?)
In regards to the combine. I think best bet would be to use that vest as a base, and build the rest of the uniform out of nylon fabric over foam. Like the kind of blue ground sheet stuff. I'm been considering making myself a faux bullet proof vest with a similar process. Making the templates out of poster board. I don't know how to work a sowing machine myself though :/, have to get my girlfriend to help me.
(she loves the Mario Kart btw)
The masks would need some customization any way you'd spin it. If you know anyone with a vaccum form table you could build up on one of those masks with clay and then pull moulds off of it for others.
Also, are you doing CP or full fledged combine infantry?
This is where I got the image, not to much detail on it though. (hella expensive)
edit: Also look up that god awful crosman clear Spas. They break easy so I'm sure there are people willing to get rid of them for cheap. Paint it up and you have the shotgun.