Try https://www.dellrefurbished.ca
Generally speaking, if Ubuntu works, LMDE will work as well. Unless you have something that is brand brand new with drivers only located in a bleeding edge kernel, you shouldn’t have any issues.
I have LMDE on an old XPS17 and it actually worked with less fuss than standard Ubuntu, mainly because of compatibility with a truly ancient wireless chipset.
I run calibre off my desktop. You can enable the Calibre content server and it can serve up your books for download (or provide a web reader).
If you have an Android device, you can use something like Moon Reader (or any other reading app that supports epub or Pdf) to download content from the Calibre content server.
With respect to covers and metadata, Calibre can tag and fill in this info as well - out of the box it will scrape information from Amazon.
Seconding Bookstack. I’ve embedded videos in it and I don’t recall anything special to do it. I also think there’s a way to comment on specific pages…mostly because I remember disabling that functionality.
Agreed on the roles and permissions aspect though. It’s pretty standard to do that for bigger deployments, but it may be a bit overkill for a single user instance.