Question about changing profession.

I know I can change my secondary profession near the end of the game. Lets say I have a monk and I want to change his secondary profession to a elementalist. Will I be able to get all the elementalist quests in the game? Or do I have to buy all the elementalist skills?

You'll have to buy most of the Elementalist skills if you've been fairly thorough with questing. There are some Elementalist-specific ones that'll still give rewards, but you won't be able to get the Elementalist skills from the quests you already completed without buying them from a skill trainer.