1. Loopable tracks. Plenty of game songs out there take one on an emotional journey. But some just aren't "game ready" and fit for certain games.
2. Number of tracks, since some games require shorter music tracks. For example, if a level is 2 minutes long, it may be a waste to use an 8 minute music track.
3. I'd rather pay for a higher quality.
4. Yes. For sure.
5. There are a lot of places to look. Itch.io is just one of them.