your pump choice will depend on how high the water has to go, not only GPH
you will need an overflow if you are not pre-drilled.
look for cracks in acrylic and glass alike. wet/dry's can/will last forever, the pump on the other hand can be a minute to a lifetime.
I am assuming your 55 does NOT have holes drilled in the bottom pane of glass, correct? if it doesnt you will need the overflow box, if you have holes this is where the PVC
you talk about is located. Ill take some pics tomorrow but I have two wet/dry's set up on freshwater tanks. One is drilled the other required an overflow box.