Now that I got pictures I can ask about a potential sump.
The area below the tank is 30x11x15". I am not sure what size sump I could get down there. Someone has offered me a 20x10x12, which is only 10 gallons but would leave room for other things on the shelf. The open space available (without going into the corners created by the stands legs is 24x12x15.
How much headroom should I allow?
What is meant by backflow from main tank, and how do I allow for it?