I'd say that as a writer looking for an artist, the more detail you can offer them, the better. That way, they have a better idea of what they're getting into. Having at least a chapter or two finished is definitely a good start!
It might be a good idea to do a trial-run once you find an artist, though? Like, give them a page or three to work on, just to see how you two work together, before you jump into the deep end?