Is clarity a stickler for you? The larger the ct weight the larger the allowable inclusions so it's possible that it won't be eye clean. The plot isn't very clean on my screen, but I would ask to see if there are any durability issues as well. If it's an AGS0 then it's already been evaluated for light performance,so that won't be an issue.
I would be getting the stone sight unseen. With a stone this size, I wonder if it's possible that it could be eye clean. Based on the image, those lines like like feathers...I'm also worried about the durability of the stone as well.
Here's a link to the stone: http://www.jamesallen.com/diamonds/H-SI2-Ideal-Cut-Round-Diamond-1433778.asp