Best idea would be to ask her unless it is a surprise and even that could turn into a disaster if you get it wrong.
I know people are different but what a man sees as sexy a woman could see and whorish and if she isnt in that kind of mood then it may end with an argument. (trust me, been there lol)
Last time i bought my GF some nice red stuff she asked me if i wanted to look like a prostitue haha
What do men like seeing them in = Lots of things.
What do women feel most sexy in = Usually not the thing you think hahaha
+1 all of this. You have said that she doesn't have any crotchless underwear; this may be because she would hate to wear it. If you don't want to spoil a surprise by asking her then play it safe and buy something similar to what she wears for you already
Stick to colours and styles that you know she feels sexy in. The last thing you would want it for her to respond with "there's no way I'm wearing THAT!"