Welcome aboard, John!!
You're asking some great questions! You will find that for some of them opinions on the value of certain accessories (such as a miter gauge) range from "highly valuable" to "totally unneeded". This exists due to the phenomenal versatility of the router, especially when one can be used free-hand, in a table, on skiis, with cam boards and many, many more ways. As a result, most things can be accomplished in several ways, and are, by different people. Some use push blocks, jigs and guides and never need the miter; others choose to use different combinations of accessories.
Routers give you a lot of "artistic expression", not only in the result, but also in the path.
One accessory you've mentioned that I do not *believe* people will disagree with is the value of feather boards for safety and easy of wood alignment, although some (such as I) have yet to need them though I can readily envision cuts where they'd add great value.
Welcome aboard the slippery slope of routerdom, where your options are nearly endless and flexibility is incredible. The only major down side to a router though, is the noise. In my opinion, getting good hearing protection for yourself and enough for any others who may be in your shop while you're working is one of the most important accessories.
Well, that and making sure your brain is fully engaged before turning on the machine!