If you've a Mac that's recent enough to have Garageband, you have no reason to look further for the time being. The point being that you record at the standard quality, which will be no different from any other similar software, the only big differences come in the question of reverb, if you want to add any, or any other treatment e.g. EQ. You can edit the camera video together with the audio from Garageband in iMovie and line up the audio waveforms (put the clips on maximum zoom) then mute the audio from the camera.
By far the bigger thing to have sorted are the mic(s) and interface, these will have a greater affect.