Well, I'd recommend skimming Vol. 1 and Vol. 2 to get a quick summary, or just find a plot summary online. I'll probably post a summarized first two volumes once I finish the scanlation, since that probably will get more people to read on.
I'd say somewhere along the lines of chapter 15 or so, so the middle of 2. That's where the over-the-top violence takes a back seat to character development and plot. Once you get more into the story, however, I'd recommend going back and reading 1 and 2 so you can get the introduction and the setup of things to come.
And volume 10 should be out by the end of the month.
I'd be willing to translate the last 4...
Please do! This manga deserves to be finished!
Contact the guys at Gantz_Waitingroom if you're willing to work on it.