For your first question, yes, you should be able to do everything you’re looking for. Your ability to edit documents online requires you to run a docker container with an office suite inside. I haven’t looked in a while, but last time I did they had some instructions for that aspect. I may look into this again to refresh myself on it.
To be honest, I don’t remember. I deleted my Nextcloud server some time after I completed the video, otherwise I’d log in and test. I may give this another shot when I have some free time.
For updates, I would just install and configure the unattended-updates package. You can set it to automatically reboot the instance overnight when you’re not using it. The only other type of update is for Nextcloud itself, my suggestion would be to log into the Linode interface, shut down the instance, snapshot it, then turn it back on. Then you can install Nextcloud updates when they’re available. You can revert the snapshot if something goes wrong. But you shouldn’t find yourself having to do this terribly often.
Anything that’s publicly available is a security risk. But you can greatly minimize that by installing a firewall, such as ufw. You can limit SSH access to your IP address. You can also enable encryption in Nextcloud as well for further security.
For email, I wouldn’t recommend making the Nextcloud a mail server in and of itself, but Nextcloud does have a mail plugin that allows it to act as a mail client and pull e-mail from your e-mail service. You can consider Proton mail or Fastmail as the back-end email service, two of my favorites.
Hopefully that gives you a good start.