Why would that make us angry?
Anyway, no, you don't have to buy a costume with it. That would be a little silly to wear everyday to school/work/whatever, wouldn't it? You're fine just getting a wig.
As for class caps, I have no idea.
Conventions for 2015 (so far...): Momocon, A-Kon, Otakuthon, Sac Anime
Cosplay line-up: Mayoi Hachikuji (Bakemonogatari), Nona (Death Parade), Nagisa Momoe (Madoka Magica)