[Deus Mortis]

But do I need all 3(+) books to play? Do I just need the one containing the army I would like? Do I just need the first one, plus my armies one?

All you need to build a generic Legion is this book: http://www.forgeworld.co.uk/The_Hor...ESY_LEGIONES_ASTARTES:_CRUSADE_ARMY_LIST.html

You can buy the other one that has the legion specific rules if you like, but it's not necessary.

I want to do blood angels, or more accurately flesh tearers immediately after their formed... So I'm counting them as having 30k rules, so I can use my favourite chapter still :so_happy:
Have the blood angels got their rules yet? If not, when are they set to?
Would the world eaters be better to represent jump pack, rage fury?
Are the rules different enough to feel... Different... More 'accurate' to a space marine?

Blood Angels don't have their rules yet and from what I've heard they aren't set to any time soon. So really you have two options; either Homebrew your own Legiones Astartes: Blood Angels rules and hope your opponent lets you use them or, as you say, use World Eaters for the time being. They capture the rage stuff quite well. But if you are going to need the one with the Legion specific rules.

Any other things I should know but can't of now?

Not that I can think of.

EDIT: how much would the average 1250pt marine army set me back?

My 1500 points list has set me back nearly £300

EDIT II: I've dug a little deeper than I had before on the forge world website and found the blood angels haven't been released yet...
And also there's the rule books the istvan legions and the crusade legions...
What's the difference? Would I need both? Would I just need the crusade book? Would I just need which ever version of the istvan book the blood angels fall into? Would I just need the blood angels 'istvan' book?

The Blood Angels will likely be part of a later campaign book, so for their specific rules you'll just have to wait.

Hope that helps.

 

[Deus Mortis]

Basically, Forge World say "ask your opponent first". I've never had a problem with people playing me though.