Go for the 550 motor as I've tried a 540 and they don't have the pull like the 550. Maybe on a 3s but that's just defeating the purpose. As you said the gyro would need separate power of some kind. Maybe voltage regulation might work. Not into it, wouldn't know.
If you can score a 4800 kv one go for it. The 4200 is still plenty but like you say too much is never enough ha ha.