When do you think the GH3 will be about $300 - 400 used?
I have the D3200 and it can way better than what I paid for it! I'd use the D3100. That being said, it's cool to say that the film was shot on an iPhone.
I'd agree with Jon Mills on using a recorder like a Zoom H1. The audio quality would be much better than a cheap shotgun, plus you can use it in the future with either a lavalier microphone or something like a Rode Videomic Pro. I use an Azden SGM-1x which I got for $60 used on Ebay. I'd try to go the used route if you don't want to get a recorder.
I shoot how to videos like this one: https://www.youtube.com/watch?v=O2kF5SJmvOg
I use a Nikon D3200 with Zoom H4N.
Here are my 7 problems:
1. Terrible Mic Input (Bad Preamp)
2. Lack of color control
3. Limited Record Time
4.Doesn't record in 4K
6. Small Sensor
7. Lack of 60fps in 1080 mode