Something to consider, and very important. The volume of water that will be removed via syphon in a failure. The Volume in the plumbing, the amount of water above the holes in the returns all that stuff. Depending on your tank size this could be huge. For me its about 1.5G. I built mine, and didn't think to much about it. I have to run my water about 2" above the pump to accommodate for this. I made the baffle on the return side to close to the pump. So I have to top off 2-4 times a day to keep everything going.
The inlet side, from the tank is normally 1.25-1.5" ID hose for most overflow boxes. Not sure about the pre drilled stuff.