It's pretty simple.
You could go with a hang-on overflow box that pulls water from your display and sends it to your sump. Then you need to buy a pump rated slightly above what your overflow is rated for.
The basic concept is that the pump returns the water at the rate the siphon pulls it out. The complication occurs if your pump is too strong for the overflow....but that's why you place a gated valve after the pump to control the flow back to the tank. (never place a valve before the pump though, that will destroy it).
Your overflow will never siphon faster than the pump, it can only match it provided the pumps' flow doesn't exceed the overflows rating.
Your next concern is power failure. When the power goes out, all the water in your plumbing will fall back into the sump. The overflow will stop siphoning, inherent in its design, but will not break the siphon. They are designed to restart automatically when the power returns. (powerhead driven ones require the PH to keep the siphon and can be more risky during a power failure) I would definetely opt for something like a www.lifereef.com
overflow (called prefilter boxes on their site).
Now the tricky part is that the RETURN plumbing will start to back siphon on a power failure. It will continue to suck water out of your display tank and overflow your sump unless you utilise siphon-breaks or have your return plumbing only just slightly submerged.
Once air is introduced into the plumbing, the siphon is broken. So.....as water is being back-siphoned out of your display, it will stop once the level drops just below the top portion of your return pipe. Or, if your want your returns submerged more, you drill a small hole in the pipes just below the surface in the display tank. The water level will drop and expose the hole (siphon-break), introduce air and stop the back-siphon.
You need to run your sump/fuge at a level that will accomodate all the water currently in the plumbing (overflow side, and return side), and amount that will be back-siphoned.
That's the "long and short" of the "long and short" of it.
hope that makes sense.