I'd suggest you not use plywood for the new deck, either as framing members or the walking surface. It will not weather well, tending to split and quickly deteriorate. A better way to go would be 2 x framing members and either 5/4 or 2 x deck planks for the walking surface (wood or composite). Planks allow water to drain through without collecting, meaning the surfaces won't tend to deteriorate as quickly, and they'll also be less slippery than solid plywood.
Use a variable-speed screw gun for making the connections, or through-bolts where necessary for the larger support structure. Unless you know what you are doing, a nail gun can easily result in painful or fatal incidents (when nails go flying and ricocheting around/into your body parts).