What you gota think about Doc, Is how fast do you wanna go, If your just looking to potter around the harbour with no interest in speed then 5hp would be fine with maybe 2hp as backup, Or if you gotta need for speed well, Check what max engine size with dealer,
As for shaft length, As mentioned do you intend taking to sea, If so in totally calm conditions a short shaft would be fine, If however you expect to encounter slightly choppier water on a regular basis, Then a longer shaft would be better, Reason being your prop will lose propulsion momentarily as you pass over the crest of some waves, Especially in a following sea, Travelling with the waves,
Also transom height and hull depth, Would determine the correct shaft length on all outboard style transoms, Sounds like you would probably be better off with a short shaft i'd say 10hp should be enough to get you up on the plane.
All the best with choosing,
