I don't know what to get for my friend who plays imperium and chaos. it should be under 75$ AUS, and i need it soon, as a battle is coming up and i won't see them for some time after
Has your friend got into 8th Edition 40K yet? Because if not you could get them either First Strike or Only War, the smaller versions of the Dark Imperium boxed game - they've got loads of Imperium vs Chaos stuff your friend would probably love. Alternatively, there are the Easy-To-Build Primaris and Death Guard kits, most recently released of which are an easy to build Primaris Dreadnought and Death Guard Blighthauler. I'm British so I don't know the prices in Australian Dollars, but I imagine the Easy-to-Build stuff and First Strike are cheaper than $75. Only War is bigger and is worth £50 here, so I don't know if that would be within your budget, but it has quite a lot of stuff in it including a battle mat and the box can be used as as a scenery piece.
He already has the starter kit, and a pretty built up army as far as I can tell, with so much stuff I barely know what he has and what he doesn't