I think that would be around halfway through volume 11.
By the way, is Claymore ongoing?
Yes. It's a monthly manga.
Well your main questions have already been answered but there was a discussion about the manga just a couple posts down currently too you could read up on http://mangahelpers.com/forum/showth...-you-recommend
I enjoyed re-reading it from the beginning though I think the anime followed the manga surprisingly well up until #11. Unless I missed something the differences are mainly verbiage or minor details, so I wouldn't say its absolutely necessary. But a must for a dedicated fan.