1. Can a Mac Pro Server be used as a Windows Server - i.e. just need the hardware?
If you have a Mac running Mac OS X Server, you already have a Server capable of supporting Windows clients very nicely indeed. (it can support Mac OS X clients as well). No additional license fees are required. The feature set is very rich.
If instead, you are intent on tossing out Mac OS X software and running Windows or Windows Server, you could do that as well.
If instead, you are intent on running Windows Server Virtualized, I expect you can do that as well.
Drivers for adapting Windows to Mac Hardware are included in Boot Camp software.