I monkied with something similar on my network but I didn't use ICS.
You basically have two options:
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Far and away the easiest method: buy a router that has OpenVPN capability. It will act as the NordVPN "client" and all PCs connected to it will route their traffic through NordVPN. You'll almost certainly incur a bandwidth throughput penalty for doing this, as the router will be eating the encryption overhead of the VPN connection.
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Put a proxy server on the VPN PC, and then connect it to NordVPN. Point all clients on the network to the proxy server.
2 is sloppy and I don't suggest it, but it can work.