Defo rtf unless you want to use an SLR or something.
Can pick up phantom 3's pretty cheap now.....depends what you want reallllllllly, Yeh you could build a quad and whack a gopro on it but it won't be a stabilised image and you want be able to control what youre pointing at etc
I do have a D3300 I could use but don't think I would put it up 100m off the ground on something I built for the first time!
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P3 Advanced is still £400+ used albeit with extras, whereas the P3 normal and the other two models are around £300 new.
I would definitely spend a bit more on the gimbal if I was to attached a GoPro to stabalise the image.
Plus, I've always been a strong believer in self builds rather than buying RTG(especially for computers
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