You want the best quality at a reasonable price, go for the Nikon D610 with the 24-85 kit lens. Excellent still photos, reasonable price, good quality kit lens that gives a good range from wide to semi telephoto and video quality as good as a dslr in this price range can give. I will admit my bias I am a nikon guy.
I am new to this photostock business, but all I can say is it is much harder to get images accepted on PD than Istock or other agencies, suggesting the quality should be highest here.
It all depends on your budget, but I would go for a DSLR, either Nikon (or Canon) and preferably with more than just the budget kit lens. The Nikon D600, D5200, or the D7100 will be wonderful and give hours of pleasure and tonnes of great photo quality. Why those two, proven performance, ability to get great glass even second hand, tons of add on extra’s.
Others will disagree of course, but that is the nature of it all
These devices can be a lot of fun. I have a slightly larger “hexicopter” also fitted with a gopro. You can get totally unique shots simply because you can fly almost anywhere. You certainly need good light to get the best quality from the gopro. And yes, jello effect can certainly spoil images with straight lines where you end up with funny jinks and waves in the straight lines. this is caused by vibrations in the flying machine. Isolating the camera using foam, gimbals, is helpful. Keeping the machine in good condition is helpful such as replacing props as soon as they take a knock. If there are no straight lines in the photo it probably is not an issue.
to keep noise low, you HAVe to use a low ISO, like 100 or max 200. Anything else will create some noise. Low light, use a tripod. Even at 200 you may end up with some noise, so programs such as Neat Image of or DFine from NIK will help you reduce the noise. Both are far better than photoshop for noise reduction
I have only joined, passed the quality test first time. I am not sure what you have posted as your trial images, but i would recommend that you don’t post any using the 75-300 lens because of possible shake, noise or chromatic abberations.
Post a variety of subjects, make sure each image is clean and with no noise – best if you submit only images you have taken at 100 iso only. Don’t try to get to fancy with filter work