Hi Jan,
There is a challenge to come up with a precise hardware requirements in the first place before your do some test drives.
Have a look on my recent blog post http://www.moodledevops.com/index.php/people-systems-and-tools-around-moodle-application
I have tried to draw a big picture of a Moodle ecosystem in an institution.
After you build the initial structure you can always scale them up as the user needs grow.
Thanks,
Faruque