On the balloon topic, I didn't mean just using a balloon to get the samples. I meant using a balloon to haul your plan up to altitude then take over powered flight from there. Since the plane wouldn't have to climb under motor power, your electrical system would only need to make enough power to make the plane cruise. That would let you use a much less powerful motor system saving significant weight, or would let you keep the weight the same and have a lot more loiter time up high if that's what you would want. If you don't need any substantial loiter time at altitude, a pure glider could make the flight simplifying the project substantially.