My experience was this. Find out what type of band she likes, gold, white gold, etc. You said you already know the cut, so that isn't necessarily an issue. I went for quality over quantity. My thoughts were that a smaller stone that has much better quality to it was the way to go. If you, her haha, want it to shine and look good go with a good quality stone instead of a big gowdy mediocre looking stone. I didn't do the broker thing, but thought about it. I went into a local shop and felt very comfortable there, so I worked with them. At the end of the day you want to work with someone you feel comfortable with, since you are dropping some coin on this. I won't say exactly what I spent, but it was in the 3-5 range. My wife loves it and we ABSOLUTELY had it added to our insurance plan. That, in my opinion, is a must. Think of it this way, can you afford to replace it total cost if it gets lost or stolen?
In the end, no one can tell you what to pick as that is something that you have to decide on. Think of it as something that you put your thought and heart into. Feel free to pm me with any questions on this as I know exactly how you feel right now. Very stressful.