You can figure that each overflow can handle about 600 gal or so per hour. Since your tank has two overflows, the maximum total flow through the sump is about 1200 gph.
This would be a typical for a tank of 175 gal. As for the sump handling that amount of flow, this depends upon the design of the sump. If your using a typical berlin type sump that doesn't contain a refugium in the middle, the 1200 gph is fine. If your using a sump that will contain a refugium in the middle you might want to use less.
My personal preference would be to use a sump that does not have a refugium in the middle, and construct the refugium in a different container from the sump. You should be able to do this since you have the 20 and 30 gal tanks to construct the sump.
I'm not sure where you got the "...I only want 3-5 the total water volume through the sump per hour ..." from, because this is much lower rate than most people run. Even the very old books on reef systems want to turn over the water in the tank about 5 times per hour. Many people today go a lot higher.
What you want to do here is to use the Iwaki md55rlt pump, at about 1080 gph, and run the output from the pump into a manifold of several valves. Off of first valve is the main return that goes directly back to the display tank. Off the second valve is the connection to your chiller and then back to the main tank. Off the third valve is the flow to your refugium. Additional valves may be added if you want flow to other devices, such as media reactors.
By using the above configuration, you will be able to control the flow to meet the requirement of the chiller and refugium, and still use just one pump. This will help keep your power bill down.
Additional notes -
When plumbing an external pump, it's best to plump it with a true union valve on the pump's intake and return. This lets you close both valves and remove the pump for cleaning or service or to replace the pump.
Shop around for the true union valves. There is a lot of variation in price. They are a bit more expensive that regular ball valves, but you only require them for the pump. The valves in the manifold can be anything that is decent.
I prefer to use both rigid PVC pipe and flexible PVC pipe in an installation, using rigid as much as possible, but using the flexible where I want a gentile curve or when I need to get around something, or I'm connecting to a pump, and want to give in the connections. I try not to use clear vinyl tubing and hose barbs because this restricts flow, and algae grows very well inside it, making some else that needs to be regularly cleaned.
When you glue pipe together, you should use PVC primer and then PVC cement. The PVC primer is usually purple. This is so a building inspector can verify that primer has been used on the connection. That isn't an issue for home aquarium projects. A clear primer is available, and it makes for a much nicer looking job in aquarium projects.