A short summary of what you need:
- an Intel Mac, a new Mac mini ($599) should do fine, any new Mac you buy comes with the OS included, if you get a mac mini you would only need to get a display, keyboard, and mouse, which I think you can use non Apple stuff.
- iOS developer program ($99 per year), you need it to test your apps on the device and to submit to the app store.
- an iDevice to test your apps, an iPod touch ($199) is the cheapest option, if you target the iPad, you should get an iPad as well ($499).
you could also buy older macs or devices on eBay for less as well, but I would recommend that you buy new stuff, maybe get an iPad 1 for less and to be able to test and target all iPad users.
about software, XCode is the official IDE for iOS and is free, you can download it from Apple Developer.
for graphics, Gimp and Inkscape are good options for free, another good cheap one is Pixelmator and Graphic Converter.
I have no clue for audio :P
hope it helps and good luck!