Do you have an emergency engine on your car? Having a spare outboard is overkill. Just make sure you properly service your diesel motor, and always carry a few extra parts like an impeller, belts and filters (fuel and oil). That diesel will run for 100 years (subscribe to US Boats if you want extra assurance).
Re; the dinghy. Engine size always depends on the size of the dinghy, and how many passengers you plan to carry. For a Catalina 30, I am guessing you would want a dinghy for 2 people, max 4. Therefore, do the math on the weight, and size an engine accordingly. I would advise against strapping a 10hp motor to your transom, as you'll easily take a knot or more of speed off your performance. Weight on the transom of any boat is often overlooked by many, and the result is a loss in boat speed.