Measure the vertical distance from the bottom of your reservoir to the top of you tank/sump where you'll be pumping the water. Get a pump that has a max head height of at least that distance.

If the max water level in your reservoir is higher then the level in your tank/sump, a siphon will be created and continue until after the ATO is off and the water levels equalize. To prevent this, allow air to enter the tubing above the max level in your reservoir.