its probably quite usual to have both an engagement and a wedding ring, However, usual dosent mean you have to do it! you could have seven rings, or none, depending on what the two of you want
Unless you are absolutelt sure what she likes, i would recmmend waiting till after she's said yes, and chose one together. Bear in mind, if you are bying her one, theres nothing to stop her buying you and engagement gift like a watch!
I would also recommend ging for "antique" (AKA second hand) as you get more diamond for your money. Although having said that, she might not want diamond