Import a list of suscribers from Excel
Try the feeds module, or migrate for importing the data. Whatever module you end up understanding best for newsletters, is what I recommend.

Your question leaves out various details: show email templates be editable by site moderators or users, opt-in or opt-out features for the newsletters. Are newsletters recurring, or sent on-demand ... etc.

I recommend looking at the various newsletter modules, Rules based modules and scheduler modules to see what fits your needs.

I find leveraging an existing solution, like mailchimp and other email services typically well worth the cost (personally).

As example I wrote custom code that looks at emails in an imported DB table and uses a server-side cron task to run a Drush command to send emails with my own custom business logic (recurring scheduled emails, if tasks are completed they no longer receive emails)... my solution is not for everyone.