Greetings

We plan to use a linux server and lamp software to keep persons info that
we must protect.
Only 4 people in our library will have access to this server via a web
based app via a lan.

Which flavour of linux server you would suggest to use?
The best way to be sure our data is safe would be to keep it without any
internet connection just to allow it to be reachable from within the local
network.
How you suggest we should proceed with having updates to the server
software but also be sure noone outside our premises could "see" our server?

Thank you for any suggestions