You're kind of jumping all over the place and not giving any guidance in terms of requirements, making it hard to give specific suggestions. If you're going to record separate tracks, then you don't want an external mixer unless it gives you multiple tracks into your computer via USB, FireWire, ethernet, etc. You DO want a software mixer on your computer that will give you the ability to save individual tracks and do a virtual mixdown later. I'm not sure GarageBand is the best tool for this.
Also, I don't recommend Apple's mobile devices because their audio isn't nearly as high quality as the various Macintosh computers have, but that's just me. For instance, get a used 21" iMac and use it as a dedicated machine for this. It'll cost about the same as a good quality 8-ch mixer..
As far as mics go, I prefer dynamic mics because they have no self-noise, although they require better pre-amps. I'm partial to Audio-Technica Pro 8HEx headset mics. The main benefit of them is they're fastened to people's heads, so the mic stays a fixed distance from their mouth the whole time. With pole-mounted mics, you need a pop-filter and the people need to move their head in front of the mic and be virtually touching the pop filter to ensure they're the proper distance from the mic. The signal levels can move around as they move their heads back and forth, which will always be a problem with some folks.