1. I think the best place would be the US or EU class forums. A lot of the guides got deleted (for whatever reason >.<) on the US side but there's still tons of information here.
2. All of those classes are good as you level, but once you get to endgame, their usefulness decreases. I've tried out Knight/Mage and Priest/Warrior, and they're really enjoyable classes (imo). All of these classes have the potential to be great, but they need a boost from the devs to get them going. But you should go with what you think would be fun. You can always grab a third class if you find out you don't like your character.
To answer your question, though, Mage/Rogue is a dpser and they're really good in PvP (either that or Rogue/Mage - can't remember

). Knight/Mage is a tanky class that has a lot of benefits if you know how to play them right. However, they are more of a dps knight than a tank since they have very few skills for holding aggro. Priest/Warrior can either be statted as a tank, a dps, or a healer. Personally, I think it's a blast because you get a bunch of added physical defense and attack from the /Warrior side so you get a tanky priest. (I was partying with a Rogue/Priest and I was taking all the hits and her health was going down more than mine).
3. You can ignore it without a lot of pain. You may want food/buffs, but you can usually buy those off of the auction house or another player pretty cheaply (if you need them at all, which I haven't)
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