A GoPro Hero 4 Silver would most likely be a good choice but they are expensive, here in Aus about $540.00. Have you already got a video camera? If so you could be filming the rest of your build and test flying with that. For flying I think you would need at least two action cams, maybe you could get away with one being a cheaper GoPro copy. There are many advantages to using GoPros and if possible they are the best way to go, stuff like using a smart phone for preview, review, control and changing the settings. I think what you are doing would make a great documentary but it would have to be filmed as you go. I think you really need 3 GoPros one on the wing, one in the cockpit and one hand held, maybe you could approach GoPro about sponsorship.