Frankly thats a good question, depending on what return pump you are using it may come with a flow control or you may have to go to a ball valve.
My sump has a pump for my main tank and a seperate very small pump for my refugium, I aimed for 80 gallons/hour through my refugium which is a 45 gallon tank.
You want to circulate your tank volume around 10 times per hour or more, your refugium should have a flow of 1-2 times its volume per hour.
If you use a small submersible pump or power head try to keep your sump/refugium above the tank, if your sump is below the tank just go for gravity feed. There are lots of overflows that will fit on an undrilled tank available just phone around to some of the LFS to get an idea of prices. Ultimately a drilled tank is the best way to go, but I use two overflows and they work fine, I think they are ugly though.
Remember to set the overflow box so if the power goes out the sump won't overflow, my returns from my sump have small vent holes so the water isn't siphoned back into the sump through the pump.
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