Given the choice of framing and other structural elements, I'd opt for southern yellow pine, which is stronger, holds fasteners better, has better rot resistance, is easier to find in good quality and generally cheaper.
Exterior grades of plywood aren't as strong as marine grades. Once you have two samples, one of each type, next to each other, you'll see why. The gussets not in the bilge could be exterior, but should be well coated. Those in the bilge should be marine. Most exterior grades of plywood will be Douglas fur which checks badly and may be a consideration, especially if they're visible.
The best fasteners are monel and bronze (in that order), with stainless being a distant third, but one that is commonly available and much cheaper. If you like rust stains, then galvanized is a good choice and they're stronger then stainless.
If you must use stainless, use 316, 316L or better grades as they'll hold up longer. If you can pick up a screw with a magnet, then it's not good marine grade stainless. McFeely's.com is a good place to start.
A painted interior is just fine, though it has limited abrasion resistance.