Hi Graham.
Re Gimbals - I currently am using the HobbyKing one that uses servos. It works really well at keeping the horizon and the camera level, what i've struggled with is very slight vibrations. Not really Jello, i've got rid of that problem by 'hard-ish' mounting the camera / gimbal and using some rubber stand off vibration mounts.
With the slight vibrations, my worry is that even with a brushless gimbal setup, I may still not get a non rock solid image. I'm currently running Sunnysky motors and RC Timer Carbon Props which are balanced on a proper balancer - which helps massively
The video is great after I run it though After Effects warp stabilizer and i'm almost happy with it. That said I don't want to but a £300 GoPro Black and a £100 ish brushless gimbal and it not be lots, lots better !!
So - I'm building a 550 frame, with 6 x Sunnysky motors and carbon props. I've gone for the Aeroxcraft landing gear as it looks like the best one out there for mounting a gimbal. Probably going to go Naza Lite with it too, then put on my current servo gimbal and see if it's more stable / better than on my KK2 450. If it's good i'll stick - if not it's then Tarot Brushless Gimbal ismy last hope. If that fails, i'm sacking it off and flying for fun without a camera !
And yes - this is my 3rd build, started with a 330, then a 450, now building a 550. I contemplated a bigger Quad, but everything I have read says the 550 is more suited to video as it's more stable. Hopefully that will be the case