i like the idea about dosing pump with two heads and one motor or a dual head parastaltic pump (dual heads on 1 motor). These is pulling water at identical rates.
for the tubing height and length, i would make it about 3 feet apart that mean the water pumped out from the tank is near the tank drain pile and the water pumped in is at the return pump area. so both lines are within the sump area.
these two lines will come out from the sump at the same hieght and the one for the pump-in water will be attached with the mixed salt berrel(may be at 1.5 feet from ground- drilled berrel) and the pump-out tubing will run to the house drain pile( will try to make it same height 1.5 feet). what left is timing device, may be a Apex, or timer device.
also to control the water volumm, we can use a stopcock or a clamp... i think... because the water volumm is depend on how big your system is and how much water you want to change daily... if the pump head can control this is an ideal.
as mention the Genesis water change system is very nice and effective but the cost and have fun to build is the other thing that i consider.
i am so excited about this project cause no more regular water change and sounds like better healther corals.