Questions as a "semi tourist" for tabletop.

Soo, as title say's I'm a somewhat of a tourist, by which I mean I'm familiar with the lore and setting but most due to video games and some audio books as well as some lore channels. I'm not however familiar with tabletop, but now me and couple of friends are planning on getting into tabletop, 40k tabletop isn't really popular around here mind you, so figured I'd ask here cause, well why not?

So what I'd like to know is:

  1. Should we get "Combat patrol" or "Kill team" sets? Which are more "newbie friendly" sets?
  2. Are Necrons "good faction" to start table top? Since they are by far my favorite faction, they are what I'd like to start with, but yea are they "good" to start with or should I start with say space marines or something like that?
  3. Can you play games other then 1 v 1 or can you play them for example 1 v 2? As in 1 player has 2000 point faction/army and the other 2 have 1000 point each? (One friend plans on playing Tyranids so, it kind of fits?)
  4. Does it matter what faction allies with which on tabletop? I mean can I as possible Necron player "ally" with Imperial Guard or T'au player, or does it not matter?

That's about it so far, I'm certain I'll have some follow up questions in comments however...depending how it goes?

Edit: Thanks for the answers to the questions! Will forward them to the rest of the friends group and we'll see how it goes for there.