The good ones are as comfy and hot-weather-ready as a long t-shirt. You'll want a dozen of them for one-and-done dressing on annoyingly hot days, so buy them at a price point where you can actually do that.
Don't worry about treating your wear-with-everything sandals too preciously. If they're going to get wet from summer thunderstorms or pool splashes or salty ocean water, they're going to be cracked and warped by September, so just buy cheap ones. You'll have more fun in them.
You'll need something to tuck into party skirts and wear underneath sheer lace tops. Silk camis are great for that because they look fancier than plain jersey ones, but still, there's no reason to spend that much more for them.
You'll want your other swimsuits to be supportive and flattering and all that good stuff, but this one suit, the one you bought for the express purpose of tanning? It only needs to be strapless and teeny, so don't worry about anyting else and just find a cheap one.
twist-front braided bandeau bikini top, $13.80, and bottom, $12.80, forever21.com
They'll become your default on it's-too-hot-to-think days. Still, summer means sweat, and sweat means yellowing armpits (gross, but it happens to everyone) so buy a bunch of wallet-friendly ones in bulk.