I presume that all you will have to do to become a subscriber is go to your account management page ("My Account") and sign up with a subscription option. I can't imagine that it would be more difficult than that. You needn't buy the game.
As for buying extra character slots, my understanding is that there was some problem with this and it hasn't made it into the game yet. I believe subscribers were to get extra character slots as well and I, at least, haven't seen those yet.
That is correct - it will just patch the upgrade in. I have all 12 of mine on all servers.
As a founding subscriber I'm good with 2 toon slots for F2P, though I'd like to see that per server instead of per account. Personally I think Bioware should revisit the Preferred Status players and bump their initial allotment to 4 (per server as well.) This would make it easier for the new player to experiment with different server types. If they find a community or guild they like they can sub to get the whole enchilada. Hard to do that with only 2 characters.