Paying for the song is irrelevant. Unless you wrote and/or performed the song, it's illegal to use it, and the composer/artist/publisher can sue you (they won't, unless you're a billionaire and never told us) and YouTube (which is why YouTube uses bots to detect and remove such vids.) The only exception to this is boring elevator music that is released for free use. You can find some of that on YouTube if you're interested.
That being said, I'm totally excited to see your vid!!!!