Yep, the overflow inside of your display tank will keep the water level at the same height. Basically, the water level in your display tank doesn't fluctuate. The place where you see the effects of water evaporation are going to be in the return chamber of your sump. So you need to keep up with the evaporation on a regular basis so that the water level in the sump doesn't drop to the point where your pump runs dry.
And the other guys are correct on how the cycle works. Pump, pumps X amount of gallons into the tank and that same X amount of gallons simply drains out and falls back to the sump.
Floods can be caused either by restrictions in the drain line, having a return pump that is too large from the amount of water your overflow can handle or back siphoning occurring when the return pump is shut off, if the ends of your return lines are submerged under the water this will cause water to drain backwards from the return line and into the sump. The safest way to avoid this is to set the outputs of your returns near the surface of the water. That way in the event of a power outage or you shut the return pump off the water level in the tank doesn't have far to drop before air can enter into the openings of your return outputs which will cut off/stop the back siphoning from continuing to drain. You'll have to adjust this yourself once you build your sump and get it running.
Edit: you posted before I got a chance. If you have an Eshopps that can handle 300g per hour then you need a pump that pushes slightly less then that. Like a 250gph pump. That being said...I think your overflow is a little too small for your system.