Cravin,
You'll need an overflow to get the water flowing to your sump..... that's the given. To avoid an overflow in a power outage, what you can do is get you a $7 marineland diffuser (or any that you can find), attach that to your tubing from your return pump in your sump.
Now figure out the level where the water will be in your sump- (this is easier to do while it's actually running) so you can adjust your slowdown valves, etc....
This is the most important thing to avoid overflowing!!!!!!! Mark a spot below the water level on the diffuser- low enough that daily evaporation doesn't effect it- but also high enough that if it does siphon back into your sump- there's enough room in the sump that it overflows. Now Drill a hole in the diffuser, this will break the siphon.