[Daox]

The head height will be the top of the water line to the top of the collector outlet pipe.

You can limit the flow, but there really isn't a reason to. Find a pump that comes closest to meeting your flow requirements and just let it flow as much as it does. You could restrict the flow, but you'd still be using the same amount of power and with the reduced flow you'd just be loosing efficiency through the collector.

The head pressure won't change as things start flowing. Basically you're spending energy to get the water to the highest height, then gravity takes over and takes care of getting the water back down to the tank. Unfortunately there is no suction effect.

Also, I'm not sure how much distance is between your tank and collectors, but the pipe between them will add additional pressure to overcome due to friction. It probably won't be a ton, but its at least worth looking at. It will also determine what size pipe you go with.