You forgot 0 flute in your question, which is my preferred countersink. Rather than a flute, they have a diagonal hole machined through to make the cutting edge. They are also somewhat eccentric ground such that the body is not quite concentric with the shank.
To answer your questions
All of the number of flutes work. It has been said that an odd number of flutes is less prone to chatter that an even number. In my opinion, any number of flutes greater than 1 is too many.
I find that the single flute work very well, I don't like any countersink with more than 1 flute. Use slow spindle speeds (about 1/4 drilling speed) and good cutting lubrication.
M. A. Ford seems to make the best single flute countersinks. These are available from Enco tools, and several other places. There are less expensive ones available, but you get what you pay for. I'm sure there are other brands that are good, but I have no experience with them.
I bought a set of Hitachi single flute countersinks at Home Depot, they were complete junk, and ground with no lip clearance. Not useable, might have worked in wood but I never tried them.
For normal use, uncoated work just fine. TiN coating might give a smoother finish and longer life, but for the home shop I don't see the need, however, the cost difference is not that great. I have been using my M. A. Ford set for the last 10 or so years, and they still work fine.
82 degree is the USA standard for wood screws and flat head screws, 90 degree is the metric standard.
