If you measure from the top of the transom or where you intend to put the outboard to the bottom of the transom, this will give you the shaft length.
With a deadrise hull, moving the outboard outboard will decrease the shaft length required.
As Xk59D says, the general rule of thumb is 1hp per metre.
I'm going through the same process and I'm bearing in mind that I'd like to use the same outboard for an inflatable tender so that may be worth considering.
Nice boat name.
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