Newbie who played Warcraft forzen thrones has some questions

So I used to play wartcraft The frozen throne III A lot. Because of this I'm thinking of buying starcraft 2, the problem is, it is in 3 parts, which makes it quite expensive, so I first have some questions about it:

1) In Warcraft, you needed to buy the game, and then could make as many battle.net accounts as you wanted (so different names). Like that you could kinda "share" your game, as you could log in with different accounts, just not at the same time as you used the same game key. Is this also with Starcraft 2, or do you buy the game, together with a user name, and are you only able to use that username to play online?

2) In warcraft the thing I liked the most were custom games, I heard that Starcraft 2 has a better engine of some sort, which makes custom games better. Is this true and are custom games played a lot, or do you need to wait quite long to start one?

3) Now about the "normal" game: I found warcraft to be quite hard. Starcraft 2 seems to be a bit of the same concept. Is it almost the same, or is there something different, that makes it harder/easier?

4) And finally: How long will it still be played? Warcraft III is quite old, and I think I still used to play online some years ago. I know the third part of starcraft still needs to come out (in 2015?), will it still be played after that for a while? Or will battle.net release Starcraft 3 in 2016 to get even more money?

4 questions, some of them might be weird. Just answer the ones you can, no need to reply to everything.
Thanks to the ones helping me! It's a lot of money for a game, as it is in 3 parts, so I want to be sure about some stuff.

5) Also one last question, which I don't think anyone will really know, but should I buy the packet of battle.net (the first and second part for +- €60), or is there an "expected" special thing which will make it be cheaper for a certain time?