iOS Mail
Settings->Mail (add or delete whatever email address you do/don't want to use).
iMessage
Settings->iMessage (add or delete whatever MMS email address you do/don't want to use. You can use ANY email address here).
MMS is the same as SMS but let's you send photos/videos.
SMS can only be used on the same carrier. AU phone number <-> AU phone number.
MMS has no carrier to carrier restrictions. It's like regular email but in a SMS GUI.
KDDi missed the boat on the iPhone. They had their chance years ago. Most people don't use AU anymore. Most AU users who want a new iPhone switch to Softbank unless they still are on contract. This is because most Japanese are on Docomo and Softbank (not necessarily iPhones), so they can SMS with their Japanese friends who are on Softbank even if those friends aren't using iPhones. I believe Softbank and Docomo users can now SMS between each other too. AU users whose contracts are about to expire are actively looking to switch to Softbank/iPhone and the other smart phones.
NOTE* You can also ignore all of the above because everyone who is using smart phones uses the LINE app domestically and internationally. It's your AIO solution complete with free voice.