What would be the minimum
hardware requirements? CPU, RAM?
No telling without knowing what all the aggregate use cases of your
usership will be. That being said, Windows is a hog and it's a poor
excuse for a multiuser application platform. Were it me and I wanted
to supply a central QGIS deployment to a user population, I'd set up
QGIS Server on Linux and be done with it. That gets you away from
having to pay for Remote Desktop Client Access Licenses (the cost of
which scales with the size of the usership) or server OS licenses.
I believe some calculations are, optionally, being ran using OpenCL if it’s available. You might want to keep than in mind. Rendering has been accelerated using parallel processing on the cpu for a while now so using more than one cpu would be recommended. I would allocate 8 to 16 GB of ram for large projects. Personally, I would not run it in a virtual environment, specially if the files are not local. You could probably get away with it for small projects.