I would say .280 is a decent average and anything above .300 is really good but I may be wrong. Also, there is a term used called the Mendoza line that sits at .200 which basically means if you can't hit over that you shouldn't be on the field no matter how great you are at defense.
I'm also not positive on the others, but it seems like if you guess the pitch right you're going to get solid contact if the timing is right (where as if you time it right without guessing the pitch sometimes you get poor results anyway, such as a pop up to the catcher) so it helps in that sense.