As Bill says, the various posts to this technical forum related to OB motors really are helpful. They influnced me to purchase a Nissan 6 hp four stroke motor.

The points you raise are valid, but power and shaft length come with heavier motors and generally ones that won't lie down in the lazarette for storage. My Nissan 6 long shaft barely fits horizontally in the lazarette. I do not imagine that the extra long shaft motors will store in the lazarette. So you might want to add motor weight, and ease of storage to your criteria.

Man, a Yanmar 16 hp in an Ariel. That certainly would be plenty of power, but hull speed is hull speed. Still, when bucking a tidal current or serious swell, the extra power would be great.