Looks good to me - same ones I use.
Rather than a single 200W, I'd suggest maybe going with 2x 100W. That way, if a heater goes out on you and sticks in the "on" position, you have more time to react before your tank boils. Or if it fails the other way, you at least have *some* heat coming into the tank.
I don't see any reason why you can't add one after a tank is setup.