Yes.
Mass Effect and Mass Effect 2 are both single player. There are no multiplayer modes
This is up to you. But, my opinion is that you should definitely play ME1 first. You get a better feel on the Mass Effect Universe, and you will be more attached to the game, and will enjoy ME2 a lot more. Mass Effect 1 is a great game, as is Mass Effect 2, but if you have the time and the money, play ME1 first.
No, the player can only play as a Human in Mass Effect 2. The player can, however, date non-humans.
If you don't import a save game from Mass Effect, you will automatically choose the character of the opposite gender to survive. If you play Mass Effect first, though, you can choose if you want Ashley or Kaidan to survive. So to play Mass Effect 2 as a female and with Ashley alive, you would have to play Mass Effect and let Kaidan die on Virmire.
You won't be able to import your save game from ME1.
It doesn't but it does have downloadable content.
No. That's a single player game.
If he died in Mass Effect 1 then he is not in Mass Effect 2. If he survived Mass Effect 1 he shows up in Mass Effect 2 but he can not be killed.
Not all of them. Maybe one or two of them, but all four games will not fit on a 4 gig hard drive.
Mass Effect 2 is a 15.
Yes, Halo 2 includes a coop mode where 2 players can play the campaign together.