The only difference in spec between DEVONthink Pro and DEVONthink Server is ‘web interface’.
I’d appreciate please explanation as to what I would need to run the Server edition and what is meant by ‘web interface’?
The web interface is what you’ll see e.g. in your default browser after opening the address of the server (see link in Preferences > Server after starting the server). In addition, the Server edition is the same app and therefore doesn’t have any additional requirements.
This is covered in the built-in Help > Documentation > In & Out > Web Sharing.