I COMMAND for older boat but E-TECH motor...have questions

[Rickmcd53]

The MWS (modular wiring system) is the name for the standard Evinrude engine to boat wiring harness. The horn referenced is the one driven by the system check tach. It will work as an audible warning so no need to hook one to the I-command. Do Not hook any other horn to the I-command as it may damage the gauge.
Purple (key on hot) wires are usually available in several places under the dash as well as black ground wires. Trim gauge and fuel gauge should both have them. Also on gauge lights blue is hot and black is ground.

[JR19]

I think I have all the info I need to do the job when I get ready...

Power for NMEA/ I COMMAND solid pruple wire at key switch, ground one of the black wires at key switch. 

Power for backlight on I Command use blue from speedomoter backlight as hot and black as ground.

The alarm hooks into my current system check tach alarm as specified by install instructions. 

I need a interface cable and a NMEA Starter kit. 

This has been an educational post that I have throughly enjoyed. Thanks you Rickmcd53 and Bluewater Bill.  

[Rickmcd53]

The Lowrance 124-69 network starter kit plus the Evinrude interface cable will give you what you need along with a few extra parts.
I do not like the idea of accessing power and ground using screw terminals on the back of your ignition switch. Those screws are very short and adding ring terminals under them is not a good practice. It is better to use studs on a gauge which have ample threads.
On the subject of the standard MWS harness, Javelin boats was an OMC company and we're factory wired for Johnson/Evinrude engines. It is very likely that instead of having the usual boat wiring for pumps, lights, etc. and a separate added engine wiring harness yours probably has it all incorporated into one harness. I've worked on a few Ranger boats and it can be overwhelming under a dash with 120 wires bundled together!