My diamond is J color. It's slightly yellow, but I don't mind. I can only really tell in certain lighting. I had seen a J color diamond in the store and it looked fine to me. Of course that lighting was better. But I wouldn't get anything below a J.
I picked my ring out on bluenile.com and my DFi purchased it. He didn't know anything about diamonds and rings.
From the website you linked, that price seems a little low.
I was originally looking at that style but when I was trying it on at the jewelers I didn't like it as much. Plus, the jeweler was telling me the 3-stone rings are supposed to be anniversary rings because they represent past, present and future. So I would be sure that that is the ring she wants.
And if you get white gold, I would have them set the diamond in platinum.
Here is my ring. It's white gold with a platinum top:
But bluenile.com was great! They are very reputable and their rings usually appraise higher than what you pay for them. The site you link just seems odd to me that their loose diamonds are expensive, but the rings are so cheap.