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.
Matsuricon/Glass City Con/Youmacon Cosplay: Armin Arlert/young Armin (Attack on Titan); Ultimate Madoka (Puella Magi Madoka Magica); Fem!Masky (Marble Hornets)