That looks awesome!
On a side note, I can't stop myself from saying that you might want to build in a safety of some type for the autotopoff.com switch, or whatever you choose. If you ever get any kind of leak in the system, it will continue to add top off as the leak drains water. One method is to use a very slow pump, such as a peristaltic pump, or put the top off on timer so it can only come on for a short while. Another option is to use a reservoir on the small side, but then you have to maintain it often.
Their switches work great otherwise.