Since your quality demands don’t seem too high I’d say you should be OK with the DAC (once its function got restored after some regression with 0.4.6 update).
But as you said you’ll need an amp, the Adafruit should do.
For the 8, 12, 16 bit issue, if you got enough space or if you decide to go with an SD card you could just map a raw 16bit into the 12 bit of the DAC.
But you might also like to look into these topics
Spark Core + Shield Shield + Seeedstudio Music Shield V2.0
Adafruit VS1053 (mp3) library for music maker shield ported?
And this port by the awesome @peekay123
https://github.com/pkourany/Adafruit_VS1053_Library