I was pretty happy with my seasonic based XFX 1K unit, and their lower end units are usually a pretty decent price point.
As far as aftermarket cooling goes.... yes you need it!!!! Stock coolers bite these days and have a hard enough time keeping procs cool at stock speeds let alone with any kind of OC applied (and you're getting a K so I'm assuming some OC is in the works). It doesn't have to be a ton of money.... if it's a budget build, then there are always budget coolers (like the CM Hyper 212+, but I don't think it's quite the darling it used to be) for well under $50, and that's money well spent.
edit: as far as cases go... you really can't go wrong with the CM budget choices, but you might want to have a look at the bitfenix cases as well... I'm a big fan of how they're marketing their cases with modding options.
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