First, don't use 304, especially if you're on salt water. You should use 316L stainless steel. 304 has a nasty habit of developing chloride ion stress cracks and failing unexpectedly.
Second, I would avoid cutting into the hull if you can. The transom on most boats is load bearing and cutting several large openings into it without a good idea of what it will do to the loads that the transom has to take is a really bad idea. If you must cut into it, making the openings heavily reinforced to carry the load around the cut areas is a really good idea.
Third, if you're going to make this, make the platform out of foam-cored fiberglass. It will be stronger and lighter than one made of wood. It will also be lower maintenance. Make the areas that the hardware go through the platform solid glass.
Fourth, it would be ideal if the platform would fit flush with the hull when it is closed. That would be the most visually appealing, but probably difficult to do. Having some way of securing it, so that it doesn't open is a good idea, but you should consider making it possible to open it from the water, since your boat is probably really difficult to board from the water without it. If you fell overboard while at anchor, getting back aboard without being able to deploy the platform might be really, really difficult.
Fifth, engineer the hinge very carefully. You're giving it a pretty long lever arm, so it must be able to take the forces involved without failing or damaging the boat in any way. The loads on that platform will be very high...think of the stupid people who will jump down onto it from the cockpit...and yes, that will happen.