Making a physics engine is a really big programming task... I don't know how much you know about flash but maybe you should start off with a smaller project before jumping into this?
Sort of the like Warfare 1917 or Age of War? That's an idea significantly easier to make work, though in order to make it a fun game you'd have to spend a lot of time refining the AI for the enemy base.
That's a good idea. Just pay special attention to memory leaks (like if a bullet goes off the screen, remember to removeChild() it before deleting the reference to it). Good luck and have fun