If it is for a night in rather than a night out: a bedroom costume, not a seasonal one. The fabric and the closures are the whole difference, and you feel it within about ten minutes of putting it on.
On sizing: read the measurements, not the size name. These follow lingerie sizing and most run smaller than street clothing. If you are between sizes, go up, because a costume that pinches ends the scene early.
If you want the character to land: one or two signal pieces. A hat, a tie, a collar or a set of cuffs does more than an accurate outfit. Accuracy is not the goal here and chasing it usually makes the whole thing worse.
If you want more than one night out of it: buy the set, wear the pieces separately. These are sold as sets, and the top or the teddy underneath usually works on its own once the props come off.
If you are new to roleplay: start with a uniform, not a fantasy. Nurse, school girl and maid are familiar enough that nobody has to explain the scene, which is most of what makes a first time awkward.
On care: hand wash cold and hang to dry. Mesh, lace and anything with trim loses its shape in a machine. See all lingerie for the rest of the range.