I notice that, even with a DM-50 mic, that sometimes in a quiet room when zooming fast in or out there will be a noise that's apparent. I guess this is normal, maybe I should aim for an external shotgun mic?

Yes, the only way to completly avoid zoom motor and handling noise is to use an external mic.

David, I'm also thinking of buying the DM-50. How would you compare the overall quality compared to the internal mic? I'll be mainly filming outdoor/nature, where I find stereo sound important.

It picks up less surrounding sound, shotgun mode is good. It's good for the price, but certainly not the "best." But very good!

I think I'll give it a go. I'm not very demanding on its performance. But I do expect to hear the difference between it and the onboard. Makes sense, no? :)