Remember you are shopping for her not you! Don’t get carried away with what you want, she needs to be comfortable and feel good!
Make sure you know her size! Be sneaky and look at her existing lingerie. It will be a number (back size e.g 32) followed by a letter (cup size e.g. D)
Consider her body shape, you might think she looks amazing but she will, like most girls, probably have body hang ups!
Pick her best asset and flatter it!
Consider hair and skin colour, pastels suits blondes and richer colours suit brunettes.
Is she a “girly” girl? Does she like lace, silk or plainer fabrics? Florals or frills?