OK, here is what I would do:
Connect all units on simnet, NSS, RC and RI. You need also a simnet power feed cable and a simnet terminator + a connection block. Everything on Simnet / NMEA200 must be connected correctly. You also need a GPS
On the Network side, I would buy a small router that can be used on 12 V, that supports DHCP reservations (static address via DHCP). I would get a router that supports USB modems so I can also get internet
on the boat when available.
Buy also 2 pcs of the Navico RJ45 adapters: Navico: ETHEXTRJ45-2MTY
You also need 3 regular RJ45 network cables.
Now, connect NSS and RI to the router with the adapters + network cables. Connect your computer with normal network cable.
Assign a reserved (static) IP for the NSS's MAC adress in the router, f example 192.168.1.100.
In OCPN, make a NMEA
network connection to 192.168.1.100 port 10110. You will now get all the data from the simnet bus from the NSS, and you don't need to buy a N2K adapter for your computer.
Start the radar plugin and you should get radar data (with heading) over the network connection.
NOTE: I'm not 100% sure how the radar handles the magnetic heading from the RC, if there is no source of variation on the simnet bus. Variation can be either automatic from a GPS
with WMM that supports it (many do NOT), or manually input if the RC supports it.
Or, maybe the radar just relays the magnetic heading and lets the plugin sort it out (using the wmm plugin in OCPN). I use true heading so haven't tested this.