Joe, personally I think zooming in and out (and panning) is way overused. There are instances where it is needed or wanted but 90% of the time I use the zoom to frame my shots and let the scene dictate the action. Constantly changing frames is annoying for me to watch but that's just my personal opinion. Changing the focul length by zooming also distorts depth of field. Best results can be better achieved by moving the camera (getting closer to subject) than zooming in. It is really hard to get good, smooth, slow zooms in camera, especially without a tripod and remote controller, and it can usually be done better in post if you keep this in mind when actually filming.
Yes, I almost always use an external mic on all my cams. Onboard mics (even on high end cams) give less quality audio than even inexpensive external mics. Since you do not have headphone jacks on your GS500 you will need to become very familar with how your cam records audio through an external mic and pay close attention to your audio meters.
Shooting video is so much cheaper than shooting film so buy your tapes in bulk and eat'em up. Your footage will be worth a lot more than your cost years down the road. Have fun with it.