If the ring has stones, check and make sure the claws are good, especially if it is an antique.
You can buy yellow gold in 9, 10, 14, 18, 22 and 24K. The higher the number the deeper the colour and softer the gold, although higher gold is apparently tempered to give it strength and it is generally made into thicker pieces. White gold comes in 10 or 14K and is more brittle, sometimes plated - be careful. White gold can't be melted down and made into something else as easily as other metals.
If you are going to invest in a ring, the setting isn't as important as the stones. Buy the best quality stones you can afford. I sometimes suggest to my friends to buy old rings and have them reset. Do get her to pick out the design though or ask a friend, they might know what she would like.
My grandfather proposed with a chicken ring, the bands found round chicken legs. He eventually gave her a thick gold band but her engagement ring was one of a kind..