Did only a quick check and it showed that the prop pitch could be too big (the Graupner 20x12 would be better) and the NEU drive could be too powerful with a 10s battery (8s would be better). The drive's efficiency would be below 50% but 18k ft could still be made with a 12 Ah battery.

Yet the whole mess has to be re-thought again. (Maybe even less pitch, maybe even bigger prop or, since there is no such prop anyway, BMatthews' idea of a twin.) After all this was only a quick check, not a thorough one.

By the way, if you define two different mission profiles it could well be that for the 18k ft mission, which is determined by climb, a low-pitch prop is best and for the 5k ft mission, if it's determined by cruise, a high-pitch prop.