2.) The Essentials are just that. Essential. The first 122 issues of Amazing Spider-Man are basically MUST reading for the character. It is as great as Spidey has ever been. The JMS stuff is a very different take on Spidey and Peter. Some loved it and some hated it. I happen to be one of the ones that loved it. I would recommend that you pick up the first JMS trade and see what you think of it before you buy any more. The good thing about his run is that it is more or less self-contained, so you can skip it if you really don't like it.
3.) I don't believe that any of those arcs are available in Marvel Masterworks. They are all in individual trades though. You may also want to check out the Hobgoblin trade that came out last year. It was very great stuff. While we're talking about Masterworks, you should know that the material in Masterworks is the same as that in the Essentials. The difference is that the Essentials are printed in black & white, they are much cheaper, and they tend not to hold up so well after several readings. If you just want to read the stories for the stories, you may want to consider skipping the Masterworks. If you want handsome volumes to keep on your shelves and don't mind paying a