OK, being at the moment a lumpy boater, I'll see if I can add anything helpful to the debate. As of last July we're running a Merryfisher 805 semi-displacement hull with a Volvo TAMD 41 200hp shaft drive (with a skeg and ropecutter, saves a few worries). Nice boat, sleeps 4, two in forepeak bunks and foldout double in main cabin. Sticking to displacement speeds (c 7-8 knots) we trundle about using about I think 1 - 1.5 gallons per hour. Get up on the plane and that goes up to about 5-6 gph at 14 - 18 kts. Missus prefers the destination to the journey, so about half and half re displacement/planing. It has a sea toilet, so discharges outside rivers/ offshore. As some upthread have said, shaft v. outdrives is a contentious subject, i prefer shaft, as generally speaking in my experience, cheaper to maintain. Draft is about 0.8 m and it sits nicely on our semi-tidal mooring on the Hamble mud. However, you will be lifting out for antifoul every 12 - 18 months otherwise you WON'T get on the plane. Fully enclosed cabin/ steering position withsliding French doors to cockpit and full cockpit canopy. Previous to that we had a Colcic 26 bilge keel motorsailer and i taught myself to sail, not difficult providing both teacher and pupil have a few braincells! Re standing rigging and sail replacement costs, we got her with the sails she had (that had been up and down both sides of the Channel a few times and were still serviceable when we sold her last July, so not much expense there. Motoring with a Beta BZ482 (13hp) we recorded a humungous 1 litre per hou at hull speed (6.2 kts) in flat water......boy, do I miss THOSE costs!) and of course free when the wind blew in the right direction. Only a brief summary but might give the OP a few ideas.
Ian