You asked
What hardware do we need?
The short answer
Ordinary computers you already own. It installs as a normal app, runs on a single office machine to start, and grows to a shared server later without redoing the work.
There is no special data-center requirement and no minimum fleet. A standard office machine with a solid drive runs a single department comfortably.
When more departments or offices come on, the same sealed records move onto a shared server on your own network, and the day-one work carries forward.
Your IT stays the owner of the deployment: your machines, your network, your keys, your backup drive.