Network without connection to Internet

Currently, I have a PC and two laptops networked wirelessly, and two PCs isolated. I now wish to set up a home server to wirelessly back up work from the two PCs not on the network, but do not want them to access the Internet once brought onto the network (no patches, updates, a/v, antimalware involvements). Is this possible, and if so, how to go about it, please. It seems to me that, without a connection through a browser, that might be automatic, but am I missing something?