Again, there is no one answer for this question.
In the settings panel, there is the option to include Email Verification or not, If verification is enabled and you do not include a value for verification on the import, the user will be able to click a button to have the verification email sent to them. If you dont enable verification, then it wont matter.
You configure the audiences when you configure the mailing.
The audience can be set to use any of the users table column values.
It could be set to send to all users whose first name is a specific value for example. your import will need to include any columns that you plan to use in the audience settings.
Your asking for specific details on a very hazy subject, I just cant give you the specifics you are asking for.
the required columns depend on you. It all depends on how you are going to configure the mailings to work.
the best advice I can give you is to register a couple of users and play with the mailings.
Get familiar with how the registration process works, how configuring the audiences works, and how sending out a mailing works, I pretty confident that once you work with it a little bit, it will become clear to you what data will need to be included in the import for the way you are setting the mailings up.