Ed..You're going through all this just to save $150 bucks once every 5 years? And going with a non-marinized solution that will cost you significantly more in the first place?
Put the plotter/radar display at your helm that gives you full control where you need it and forget everything else...it is just added complication and non-integrated stuff to go wrong. You want redundancy...put seaclear on your laptop and wire it up to your
handheld GPS...want more...get mapping on your
handheld built in. There is no point in a redundant screen for
radar since the most likely failure will be on the mast.