How prepared will I be to play Arkham Horror after reading just the Rulebook?
The best thing would be to learn the rules from someone who's very proficient in them, from their own experience playing. My experience, however, consisted of learning to play Arkham Horror with four friends who were all new to the game as well. In that case, I would suggest that anyone who has not played the game before at least attempt to read the entire rulebook and FAQ first. 

If you don't have at least one person participating who's very conversant in the rules, then you'll probably be referring to the rulebook a LOT, not just checking the FAQ for clarifications. Note how long the "short" version of a quick reference guide in this answer is. It's pretty hard to explain the game concisely, and a lot of complications arise beyond the basic rules of the game. Naturally, the FAQ derives from those complications, and simply being aware of which characters, cards, etc have clarifications written about them will help you know whether to consult the rulebook or FAQ while playing. Don't expect to remember everything you read, there's just so much, but do everything you can to make it easier to refer to later.

If you consult some other questions on Arkham Horror, you'll see some comments about the importance of at least one player having the expertise to guide other new players. If you don't have that one expert player, then simply reading the rules AND the FAQ may not be enough to get everything right when first playing. So in short, you should read everything, and still be okay with the expectation that it may take a while to get it all right!