I'd aim for level 50 and decent gear rather than necessarily "best" gear right before an expansion. The current stuff will be outdated with newer expansion stuff anyway, so whether or not you have the best (or at that point former best) likely won't be that important anyway.

There's still quite a while before the expansion comes out, so if you rush through the way most people do, I doubt you'll have much trouble getting to 50, getting some good gear, and finishing the storyline by then.