There's lots of purists who would never mount an outboard on a transom. I'm not happy about it, but I'm doing it this summer (on a Caliber 28), as the diesel seized and I just haven't the money to repair
it. I'm having a canvas engine cover made, to match the other canvas. That way, it will look a little more integrated. (If you have a three-blade, remember to pull off the inboard's prop.)
Check out the Garelick 71090 and 71091 motor mounts. They're the toughest in the Garelick line
, will hold four-strokes and you can change the angle.
For an engine, look at the four-stroke Yamaha T8 25" shaft high-thrust prop, engine. I've had it on another boat for two years, and it's awesome. It has elec start and elec tilt, though even with the Garelick lift-up feature, I had difficulty getting the prop to clear the water for storage.
It has a massive (for a sailboat outboard) 12" 6.5 pitch prop, total balls. Forward, back down, whatever you need, it's there. Remember, for a sailboat outboard, you don't need HP, you need thrust. You'll never be able to max out the top speed, you want instant thrust for turning, backing down. If you're choosing between an 8 or 10 and a 15, buy the 8 or 10 and invest the difference in a high-thrust four-blade prop (mine cost $175). It makes a huge difference.
Though you might not have a choice (money) try to mount a shifter in the cockpit. It makes harbor work much easier.
Look at the Bristol page, I did a long response to a guy's interest in auxiliaries.