DING DING, buy something you guys can afford, she will appreciate what ever you get her, as the main joy of this should be the marriage, not the material item on her finger. If that is important lol then might want to re think it.
I bought my wife what I could afford at the time she loves it cause of what it means, at the 5 or 10 year mark I will go and get her some bling bling piece to have.
Another thing is go in her jewelry box and grab some of her rings to get a size for her, that's the easiest way with out asking her what her ring size is.
As for what to style to buy her good luck, I got lucky my wife likes thing to be very simple.