It seems like you are pretty much set. With that budget and two options for manufacturers, there are only so many cameras available new right now. Do you want to buy the camera locally or online?
If you don't have anyone to help while you take photos of yourself in cosplay, then you should consider a camera that has a "fully articulating" LCD screen on the back. This will allow you to see yourself on the LCD screen ,so you don't have to do trial and error photo taking. Although, it would probably be difficult to find a new camera with a screen like that for $500 or less. The Nikon D5100 replacement was just announced, so that model might get cheaper until they stop producing it. Maybe the "Black Friday" sales coming up this month would be a good time to get it cheap.
Other important things to get:
1. A tripod, take note of how tall the tripod can get when fully extended. Some cheap tripods can only get as tall as 3 ft or 4 ft.
2. A wireless shutter release (a remote control for your camera). This will allow you to fire off the camera or start the timer from a distance.
Both of those items will help you to take photos of yourself in cosplay without any assistance.