You can buy the USB SuperDrive made by Apple which will match the aesthetics of the iMac, or else there are other less expensive third party alternatives.
i would get the blu-ray. never know when you may want to use it. there's some good free and paid software out there for it. i forget the name, it is home on my macbook's bookmarks. but best buy as a well rated player/ripper for not too much $$