You might be better off in the long run going with MacPro hardware. You can buy a “family license” of MacOSX to run it on 5 computers, however, not that I’m encouraging it, but you do not need a serial# for Mac OSX as you do with Windows.
If you have any Windows software that you absolutely need to run, which does not have a Mac counterpart, you can run Windows on the Mac now as well, either within a window in OSX or boot into it.
Just a suggestion, but certainly one to look into.