I would imagine that 10hp would be adequate for pushing your boat at 3-4 knots in calm conditions. Offset installation shouldn't make any appreciable difference for trolling. Why don't you want to use the diesel?
Better yet, why not sail whenever possible?
I do sail when appropriate, but sometimes due to lack of/direction of wind or confinded spaces I need to use the engine. I don't really like to use the main engine because it goes to fast most times, which means in and out of gear every minute or so. And when its not to fast it's just at an idle which I don't like to leave it at for hours.