1. Calls to the USA and Canada from anywhere are free.
2. & 3. Downloading Hangouts doesn't give you a phone number. You'll have to sign up for Google Voice (free) if you want one. If you set it up, it's free to pick up any calls wherever you are.
You can make calls in Australia just as you would stateside, but you'd be charged at whatever the applicable per-minute rate is if you're calling a non-US/CA #. Receiving calls is free but if your caller is from Australia, they'd be stuck with paying long distance fees to your US number.