Basically... NMEA 2000 is the way you'll need to do it.
This is a network system, think of it like a tank of water, any pipe can put water in and any pipe can take any water out. Everything is shared.
For a basic system all the information you need is the engine data, and a GPS module.
The iCommand can show all this data but you can also link your plotter into the network, and assuming you choose a suitable one, it will show the engine information too which can be very useful for example in order to store fuel data.
I think you need BRP P/N 0763541 which should connect the engine's brain into this new network, I would suggest a call to Mike Vincent at South Coast Marine to have a chat and make sure which parts you need, he will know if you give him your model year and say you want to connect it into the NMEA 2000 network. This cable(s) will need to be run from the engine up to the console. In the console you will run this new cable into a "t" connector. See here
for a really good basic guide.
To this you need to add a wire which runs to the battery, then you need to add in a GPS module, probably the new Lowrance Point-1. An iCommand then just has one wire which goes into the network and it takes its power from the network. If you have a compatible plotter it will get position info and engine data off the network too.
To begin with you need:
NMEA 2000 terminators (Male & female)
4x NMEA T connectors
NMEA 2000 power connector,
NMEA 2000 GPS Module
NMEA 2000 harness for the engine
I would suggest you also get the Evinrude Memory Module, this allows the iCommand to calculate the fuel you have remaining just from the fuel it knows you have used.
I recommend Garmin for the new plotter: 751