Okay, i know just enough about this subject to be dangerous. Hopefully a little bit helpful in there as well. Here's some random thoughts.
1. Recording in camera is super convenient and will probably work just fine for your application. Most of the cameras you will consider for this will have this capability. Invest in fast, large SD cards and you'll be fine.
2. So long as all your video files have some kind of audio on them [distorted on camera mic included] FCPX will automatically sync them for you along with your remixed audio file, which is super handy. You can also run all your camera files 'live' as if it were a live show and 'cut' between cameras as if it were a live show and FCP will build your initial timeliine off your real-time cuts. This is a SUPER fast way to get a rough cut of a concert. You can then tweak those cuts until your eyes melt if you really want to. [note: doing this 'live' cut editing does require a good bit of processor power so YMMV]
3. The BMD cameras are crazy good for the money. The image quality is exceptional [again, for the money]. I'm not sure you can get 4 Micro Cinemas with lenses in your budget, but using one with a decent lens for your main money camera might be worth considering.
4. GoPros have a tendency to overheat if running higher resolutions for longer periods of time. I'd do a little online research before you buy those for this application. The newer ones might be better, but my Hero4+ is useless for anything 1080p or higher longer than 5 minutes or so.
5. An audience shot for cutaways is a SUPER handy thing to have. Before and after show B-roll is also a great thing to have during editing.
6. Changing just 1 camera to an operated one will significantly improve the watchabilility of your final product, especially if you want to produce clips longer than 90 seconds or so. The operator doesn't have to be on comms or whatever. They just need to be instructed to get an interesting shot, sit on it for 90 seconds or so, then get something else interesting. This can turn one camera angle into dozens of usable shots for later editing.
7. Ambient mics at stage AND FOH can be very useful. Just time align them...
8. You may want to consider TWO cameras at or near FOH with different shot composition rather than 2 side stage cameras [ maybe one operated see note 6 above]. Or something 45 degrees off center if you can get there and have the camera live through the experience.
9. A few 'shaky cam' cutaways taken on a cell phone can add to the excitement of your final product, especially if they are 'mid-crowd'. Again, FCPX will sync whatever short clips you may take based on the audio so don't worry if they are just short little bursts.
It's official. I know too much about video...
That's all i got for now. I'm sure i'll think of some more things later.