I'd have to bow to those that know more about how propellers are affected at lower altitude density. But consider that this is pretty much a climb then glide style flight pattern. To reach up to 5000 to 15000 feet with this craft about half or more of the flight time will be spent in the climbing. So for THIS case it is the climb phase rather than the cruise phase that will be the mode that shapes the design of the craft.

There's been some mentioning of the idea that there may already be suitable craft available from other universities or research groups. It would certainly be worthwhile looking into what they have already.

There is also a lot to recomend the idea of twin tractor motors over the idea of a single motor pusher. The tractor motors could be out on the wings where they can still have a good size propeller diameter for good climbing efficiency while keeping the nose of the fuselage clear for sampling intake openings and camera lenses. Such a configuration also gets away from the rather cumbersome twin boom layout that is needed for a larger diameter pusher prop. I know it doesn't mattr much when at the field or in the air. But at some point you need to pack the whole thing up and take it back to the storage area. It also reduces the need for additional strength in the tail surfaces since they are no longer a structural element that joins the two tail booms.