As 400 operating system os downloads.
I adopted an old as 400 from ibm headquarders in rochester mn. And they gave me the software and manuals and units.
They use a box that has keyboard, video, printer and ethernet connections. It links to the as/400 unit via a cable called. Twin x connection. The software links are here in iso format for the as/400 version 4.0
type 9402-2xx an as/400 server box not far from what you described.
If you want the software download links contact me at digital.insanity8 via gmail and i will send them. No cost just to help.