I would've gone with "prebuilt servers" or "rackmounted servers". Those are of course simplifications, but would make it a bit clearer that it's referring to specific server hardware and not servers entirely.
That would sound either like they are selling custom servers or kits, or they are selling servers in other form factors like towers which both Dell and HP have.
We are going to have a Dell PowerEdge review tomorrow. We call that a server or more than one servers not server units so that would feel strange to me.