Chapter 1: Because in the Beginning, There is Music...
1) The size of the music files against the capacity of your player. If you rip music in extremely high bitrate or in lossless format, you might not be able to fit as many music in your player when there is limited space.
2) The format your music is in and the format that the player will support. This is mainly about how conviniant it will be for you to store, manage, sync and play the music. If you want to store lossless format in PC (as digital archive), than convert and sync music in low bitrate, that is fine. However, you might want to consider a high lossy format so it will work well for both PC and portable playback. Of course, choosing a good software to mange the music is important too. Remember, you will replace your DAP in the future, but you won't want to rebuild the whole music library every time just becasue you change your player.
3) Whether you can differentiate between format and bitrate. If you can't tell between 256kbps from 320kbps or lossless, or maybe .mp3 and .wma sound the same to you, than you might want to pick a good enough bitrate (not too low or high) that you find good sounding, and a format more widely accepted. In this way, you ensure you can still get good music and you don't need to worry too much into compatibility on future upgrade. In that sense, I can tell you right now those low bitrate music you bought online will not likely to survive to the next decade. You will be better off with high bitrate or lossless in DRM-free format, maybe even some old good compact disc.
To Part Two.