100 A is more than plenty for what you describe...maybe 60-70 will be better and certainly cheaper and more readily available. I'd go with the 3-wire alternator, running the "sensor" wire to a "central" location in your wiring system where everything gets its voltage from. I use a voltmeter (measuring the "central" voltage) and I also hooked up an LED alternator light in my customized instrument cluster.

You can do a search on the one-wire vs three-wire issue, lots of good articles.