Hi and welcome, @red2.
I believe this is possible, by first downloading the discourse program to a local computer, then it should be possible to burn that image to a disk or move file to an external drive and use that for the install.
You may not need a domain name + mail service for that, am not certain.
As for question about if Discourse is ideal for your use case of an internal communication tool, that would depend on how you would plan to use this and what alternative options are.
I wish you good luck and good fortune with this!