That style of engine mount has been around for quite a while, and I've never seen one fail due to the design itself. You should check the screws once in a while, and it wouldn't hurt to locktite them when you install the engine, but no matter how you mount the engine, it always comes down to screws that could vibrate loose.

On the prop, I'd say try an 11x5, it will give you better climb and a slower straight line speed. Though for most training it won't matter that much, you won't be spending much time at full power. However the extra climb of the 11x5 could be useful when aborting a landing approach.