The devil in the details
Well...to take your questions in order:1) If I have the money for the larger screen, I would go that way: At the helm you're trying to read it while dodging suicidal PWC Operators, hailing the oncoming frieghter, and steering the boat...amoungst other things! Having a large display makes it easier to find the information you need. Also, I tend to have a bunch of insrtument data displayed on the screen too (depth, SOG, COG, wind speed true and apparent, etc) and those little boxes take up space on the screen! On the other hand, the larger screen doesn't leave much space for anything else so you need to think about your cockpit layout...Where you mount things is going to be different if you race around the buoys, or daysail, or make long passages in nasty weather. I tend towards longer passages and hence my wistful comments about two chartplotters. For daysails or coastal hps where you are almost always driving from the helm, the one big screen makes more sense. For racing, you might actually want it below decks so the navigator can stay out of the way!2) I have to confess. Although I've become a serious DIY sort of guy since then, the pedestal/chartplotter installation was one of the first things I had done when we bought the boat and I paid the yard folks to do it: I don't know if the holes in the cockpit sole were already there. If they were, though, I'm sure they had to be enlarged! You'll be amazed at how many wires/cables run up there...3) Yes. Definately run new power to the chartplotter, especially if it's also doing your radar. Follow the installation recommendations about wire size (and go larger if in doubt) and run it all the way back to a dedicated breaker/switch. The chartplotters are not very tolerant of big voltage transients. I actually have all of my electronics on an isolated, filtered power supply that maintains a very steady output voltage independent of the battery voltage and filters any spikes out. Its probably overkill, but I haven't had any mysterious electronics issues since I went that way...