Wait, if you know how to swap a psu (and therefore all the cables), then what's the point of buying a prebuilt in the first place? That's going to cost you extra money, not just from the fact that you're buying a prebuilt, but buying a prebuilt only to swap out two of the components.
If you can't figure out how to select components for your PC, I suggest beginning with a
DIY combo deal. If you want to go semi-custom, choose a combo that doesn't have the full complement of PC components. Also, if you want to get a rough idea of what your PC might cost/some ideas for your build, Tom's has
several configurations readily available for review.
Once you build your own computer, you'll never turn back to prebuilts. It's NOT difficult, and if you can't intuitively figure out the assembly, your mobo will have a manual (as will the chassis.)