First off, if she wants the look of white gold, go with platinum instead. I chose white gold for my engagement ring, but if I could do it all over again, I'd do platinum. The rhodium plating (that makes the white gold look "white") wears off the back much faster than I thought it would. Gold is actually trading higher than platinum right now, so there shouldn't be too much of a price difference I think. The head, however, should always be gold.
Second, get your terms right. A solitaire is just that--a solitary diamond, no side stones or melée (small stones in the band). If she wants stones in the band, don't go looking for a solitaire setting. The side stones can be channel set, prong set, or pavé. If she has a preference, make sure you go with that.
Third, the 4Cs have somewhat of an order of importance, but they're dependent on each other as well. For example, I think cut is most important. You (and she) want the diamond to really sparkle. Make sure you do not get a shallow cut diamond. The depth should be about 60% of the overall width. Shallow cutting is common because it makes the diamond appear larger, but it has much less brilliance and fire. Next I think is color, but only in so far as it's an H or below. No one is going to notice if it's a D or a G when it's on her hand. Clarity matters in that you don't want any visible flaws, nor do you want flaws that affect the brilliance and fire. But as others have said, no one is going to look at it under a microscope when she's showing it off. So get a good balance of those three in the largest size you can afford, if that makes sense. Be aware that a .98 carat will be significantly less than a 1.00 carat just because of the jump at the carat mark, but no one will ever know the difference.
The downside of BN is that you can't actually see the stone before. Two stones may have identical clarity ratings, but their actual flaws will cause them to look different.
As far as amount, go with what you think is best. The "rule" about three months salary or whatever is garbage. If a woman doesn't want to marry you because you didn't buy her a big enough diamond, or spend enough money, she's not the woman you really want to marry.
Finally, when picking the ring (including the size of the center stone) take into account the size of her fingers. If she has smaller fingers, the stone will look bigger, and vice versa. Also, if she has smaller fingers, a thinner, more delicate setting will probably look best, whereas if she has larger fingers, a thicker setting will probably be better.