Only in the Late 2012 iMac. If you have an older computer, you have to use a DVD.
To get the Windows support software, open Boot Camp Assistant (in /Applications/Utilities), mark the box to download the Windows support software and follow its steps. You will have to install the Windows support software after installing Windows.
Make sure that the Windows you want to install is supported on your computer > http://support.apple.com/kb/HT5634?viewlocale=en_US&locale=en_US Follow the Apple's steps > http://manuals.info.apple.com/en_US/boot_camp_install-setup_10.8.pdf
Yeah you can. I did it about a month ago. This is a good guide that shows how to bootcamp: http://appducate.com/2013/03/how-to-install-run-windows-on-a-mac-with-bootcamp/
And you can get the bootcamp drivers from Apple's website anytime. Here's a link to them: BootCamp Recent Software Updates
You can also download the drivers once you are using Windows on your boot camp.
Yeah you can. I did it about a month ago.
Read my comment. You can only install Windows on the iMac with a USB drive if you have a Late 2012 iMac. The rest of iMacs come with a SuperDrive and the firmware doesn't allow them to install Windows from a USB drive.
Also, Boot Camp Assistant already gives you the option to download the Windows support software, and it's not recommended to wait to download it after installing Windows