I'd rather buy her a car, even if it is 2x, 3x more expensive than a ring. At least it has some utility and it definitely has a wow factor to impress her friends with.
The expected lifecycle for a ring can span generations (or used to, at least). A car will last you a decade and you'll be eyeing others in half that time.
If this kind of gift is made to be symbolic of your relationship, then a car (or any other heavily depreciating asset) isn't the best symbol.
One of my friends got an olive tree for her 25th wedding anniversary. I thought that was a great gift.
The expected lifecycle of an engagement ring is one engagement. Getting her something "used" would be even worse than something "fake".
And diamonds are a heavily depreciating asset. A car loses about 10% of its value the second you sign the purchase contract. With a diamond ring, it's more like 60%.
If this kind of gift is made to be symbolic of your relationship, then a car (or any other heavily depreciating asset) isn't the best symbol.
One of my friends got an olive tree for her 25th wedding anniversary. I thought that was a great gift.