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Sony Movie Studio is an excellent choice, you could save $600 and get pro but then you might as well get Abobe After Effects for 1k.

freeplaymusic.com has amazing tracks. I use some of there songs the only pity is that you have to download them to preview them 🙁

by externally powered, is the mic picking up and recording on a seperate track than the video, or is the audio saved with the video.

You can mix your volume in editing software, Sony Vegas, Adobe After Effects, they all have some form of audio mixing capabilities built into them.