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!
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)