If you want the biggest change for the least money: a harness over what you already own. It restyles a bodysuit, a bra top or a bikini you have worn before, which is more outfits than buying another piece gets you.
If you are buying wings too: buy the harness in the same order. Wings hang off a strap set. Working out on the day that you are short a mount is the version of this that ruins a morning.
On sizing: underbust and shoulder, not waist. These adjust through sliders and most run one size across a wide range. The straps that decide the fit are the ones over the shoulder and under the bust.
If it is going over a printed outfit: plain straps on a plain base. A harness reads as a shape, and a shape needs somewhere quiet to sit. Over a busy print it just disappears into the pattern.
If you are in it all day: wear it in first. Vegan leather runs warm against bare skin in a crowd. An hour at home tells you whether you want something underneath it, and the answer is often yes.
If you are new to the look: start at the leg. A leg harness over fishnets or bare skin is the lowest-commitment version of the same idea, and it works with an outfit you have not changed at all.