Hi guys,
The onboard e-compass is good enough and you won't obtain much more accurate readings if using external one, but as there are many good GPS modules with onboard 5883 or 5983, it worth checking out.
What about the GPS modules, I have very good impressions from NEO-7M too, more it is cheaper. Just choose the ones with larger patch antenna - 25x25 or even larger one - 35x35.
@magicman - The AUAV-X2 uses the original PIXHAWK firmware - stable or master and can be programmed via QGC. It can also use APM via MissionPlanner. The hardware has different power supply, but other changes are minor.
Best regards
Nick