This is the smallest overflow box that I know about, its rated for 75 gallons but I think that oversizing is always better then undersizing! The max flow is 300 gph so you will need a comparable pump. Hope this helps!
Yes, and in case of a power outage, it will automatically restart the siphon. I believe that the CPR overflow boxes are the only ones that use that; the other brands start automatically (which is a plus).