I am teacher and I would like to make a NAS so as my students could long in and watch or download documenters, films or study material. Which is the cheapest model of WD NAS appropriate for this purpose?
I will let others inform you about a NAS, although there is another way to view excellent documentaries. See this link https://curiositystream.com/ Free trial available, I have an account which I enjoy very much. With just an iPad and Chromecast device on a TV you can view them on the big screen.
You can visit the Learning Center and see what all is available and decide what you feel you need and will work for what you want to use it for.