4 things, yes they are needed.
First: The suit. I tell people about the scarface costume all the time cause it is pretty much nick's suit, though the material is pretty cheap. still it's an inexpensive option and pretty authentic, just switch up the undershirt and your good.
Second: Left 4 Dead stuff. We are talkin heath kits, defibs, adrenaline shots, pillz, and other stuff. without essential props you can be confused with many other people in both anime and video games (or for scarface)
Third: Weapons. this part is tricky, since a lot of cons do no allow airsoft weapons even if they are gutted. If they do, I would look for a boneyard gun or an el-cheapo airsoft you can gut. As for safe props, you can do a shotgun pretty easily with some insulation foam, some pvc pipe, a foot scraper and paint. made one before, not too bad just a little fakeish.
Forth: attitude. Nick is...well an *********. he pokes fun at ellis an coach, hate's rochells optimism, and only really cares about himself (I know he shows some emotion when he looks on the corpse of a dead comrad, but most of the time he is just mean). poke ellis when he is in a story, tell coach he does not need another hamburger, and generally don't show interest in the others. just act like you want to get the hell outta there, and the rest of the team is slowin' ya down.