I just happened to review this thread and realized I didn't answer your questions.
To get a @icloud.com address, all you need to do is set up an Apple ID.
Be sure to carefully record the password and security questions.
You can have as many Apple IDs as you want, but don't go overboard, as that can get confusing.
It's free, and comes with iCloud storage. What, exactly, iCloud includes is changing over the next few months. But we can say that it will include photo storage and availability on all devices, as well as file storage and availability on all devices running the right software.
It's an IMAP e-mail account, which means you can access it from any device using the same Apple ID.
Note, however, that if you move an e-mail from the inbox on your Mac to a folder (mailbox) in the On My Mac section, that removes the e-mail from iCloud. If you want to keep mail available on all devices, set up folders in the iCloud section.
I have an @mac.com e-mail address that I use on my iPhone and my Mac. My wife has an @icloud.com address used only for her iPhone.
In Mail on your Mac, you can manage as many e-mail addresses as you like. You could also have multiple e-mail addresses on an iPhone or iPad.
Note, however, that you need to be careful to note which e-mail address you're sending from. It's easy to respond to an e-mail and send it from the wrong address.