No, there is no limit to the amount of content you can store on your CMS. The uploading process will draw on your monthly bandwidth allocation so this is something that you have to consider during your website development.
It is possible to import external email addresses into your database via an excel spreadsheet as long as all fields match your existing fields in an export list
You can address your subscribers in the copy section of your email campaigns by adding a special code in your text which will automatically be replaced with their name/email address