[Faster]

8 - 10 HP should be plenty unless you are having to regularly fight strong winds and currents to leave your mooring/marina area. The 4 stokes in this power range have a lot of push, are much quieter and burn cleaner.

A long shaft is certainly recommended for a transom mount esp if you are having to motor in sizable waves (as I'd guess you might). Not sure of your setup, but a lifting mount may be indicated if you would otherwise drag part of the motor in the water when sailing.

That 50 hp motor is totally not what you want!

 

[Faster]

Stay with the 50 horse. It should weigh enough to make the stern squat down enough to keep the prop in the water. On the days with no wind you can go water skiing. I think that the extra weight of the 50 should only cost you 1 to 2 knots of sailing speed.
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Sometimes I think you're just evil, bubb!