To answer your question, "Do we establish an Internet account or sub-contract for an intranet setup?" you should do both. Windows networking has become so easy that anyone could set up a network -- but setting up a network that meets your requirement for security requires an expert. Regarding the hardware and software you'll need, I would recommend setting up a Windows server system. Use Windows Server 2003 Standard Edition, if it's available when you start building your network.
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