honestly, i think an old beat-up gas mask would make it all look much more grimdark. armor plating, I would go in for a cheap plastic trash can at wal-mart, cut plates out of it, rivet them on the costume. As for weapons, if you want to be Khornate, I think a chainsword/ axe is necessary. That could easily be done with wood and plasticard to pretty it up. Same for the bolt pistol.
And, to top it all off, make it all chaos-y, lots of tattered belts going across the torso.
"You all did see that on the Lupercal
I thrice presented him a kingly crown,
Which he did thrice refuse: was this ambition?
Yet Brutus says he was ambitious;
And, sure, he is an honorable man."