But all the wires I need are still there right, so I don't need an extra few bucks to buy more wires, correct?
That is correct.
Both fully modular and semi-modular PSUs include all the necessary wiring needed to run your PC, Non-Modular just comes with it all attached, and sometimes in the way (internally and permanently attached.) The only reason you'd have to buy wires is if you need a custom cable.
just be forewarned: cheap junk can come in any of the three, although mostly seen in non-modular. The CX-550M is above that junk, even if it is the builder series line with its short warranty and tendency to die right after the warranty expires if they're pushed hard. Hopefully yours is newer than the Builder series with a better warranty.