Synology DS214se

You can set up different folders (I have movies, TV, Kids and christmas) and just plonk the files into the relevant place. In kodi you need to set up each path in the menu and fiddle with other settings.
Personally I prefer Plex as I find it works a bit better; for example in kodi I found that movies would always update as new stuff is added but the TV shows were a bit random.
Incidentally you can have the best of both - use kodi but with the add on 'plex for kodi' and the plex media server.

 

Anyone you give permission will be able to access the server. Usually this is you as administrator plus family if needed.
I use Plex media server installed on the nas. If I had a better Nas then I wouldn't use Kodi at all. As it is, my set up works best using Kodi as the player and accessing the media server. I use a video add on, installed onto to Kodi, called 'plex for kodi' and all it does is make Kodi look and function as Plex does. Entirely personal choice.
You can look at alternative media servers and players (emby for example)
Plex uses something called a Plex pass with various payment options. I don't have one as I only use it on my android TV and it works fine.

 

You really only need a VPN if you're downloading stuff that perhaps you shouldn't, or if you are particularly worried about privacy. It doesn't affect the nas as it's basically just a local network. Any remote access to the nas (eg you suddenly want a copy of a file stored on your Nas but you happen to be on holiday) should already be encrypted and certainly it will be password protected. As a rule, hackers just aren't interested in Joe Public's files, they'd be more likely to try and hack passwords from Google etc

 

I just ripped my dvd collection and put the files on the nas. Freed up a lot of shelf space!