Just my two cents -
I stick to the pre-rendered video. Aftereffects(and premier) has a timecode function, I make one 5 minute one and use it for everything with different in times per cue.
I've had repeated issues with using images in a comp for this. I'm not sure exactly why, either.... The images never seem to play back the way I intend. That being said, I never gave it much effort as I found the other method just as easy.
But yeah, countdowns would be great !
Z