Expect enough candy to put you into a diabetic shock.
The modern vibe of Halloween in America is to be someone you're not for a little while. And just about ANYTHING can be a costume. In October, you won't have any problems finding a costume to buy or you can re-purpose clothes you already have. You will find costumes in just about every store (Wal-Mart, CVS, K-Mart, etc.) and even a lot of grocery stores.
For example:
Redneck - wife beater t-shirt (sleeveless T) and/or plaid shirt. Cigarettes in the pocket. Stained/frayed blue jeans. Beard stubble. Baseball cap preferably with beer logo. Cowboy boots.
Nerd - Pants pulled up to your armpits. Button down shirt buttoned all the way up. Glasses with masking tape wrapped around the middle. Snort a lot.
Farmer - Overalls. Safety pin stuffed animals all over. Straw hat.
White trash - large clear trash bag filled with styrofoam products.
Coming all the way from Australia, I'm not sure I'd want to invest a lot in a costume I'm only going to wear once. There are a lot of things you can do with clothes you already have that may only require one or two small "props".