Cuervo,
Yes, I will have a valve. I was actually planning to use gravity feed for the water change. I will use a ball valve on the sump tank exit positioned very low to allow the sump do be nearly drained. The exhaust water will go directly into the house septic plumbing (through a trap under the house to prevent any fumes from coming back up just like any other waste drain).
One of the key goals of this build is to get everything as low maintenance as possible. I'm lazy at times, so I need everything easy to help me be successful, plus that's one of the fun parts for me, all the details to get things working together. I think the water change system is one of the big issues with this, so I'm hoping this turns out to be a good solution.