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Rest are clothes/weapons/cheats/rubbish.

Thank you and Reverse Cosplay.

You don't need anything else.

Get the "Thank you" and "Reverse cosplay" packs if they didn't already get added automatically - those are free anyway.
For the rest - whichever costume packs you think are funny. I'd recommend the Dubstep Remix pack, because killing fools with Polka music is more funny than doing it with wub-wubs

Nope. Season pass is pretty much all you need.