A 550 or larger hex frame build should be large enough for whatever size camera you choose, have you considered that?
We make a variety of ready to fly UAVs and frame kits that might be suitable for your needs. I have attached a picture of two of our frames with a 3DR IRIS for scale. The large frame supports 30" props, while the smaller frame supports 18" props. Both frames are extremely stiff and lightweight, and are sealed from the elements.