A refugium is not necessary, but it's good to have to help with filtration and allow critters to breed without being eaten. The 55 is the biggest you can fit. The skimmer you are going to need will go in the first section, the refugium will be in the second section, and the return area will be the third section. Between the second and third section it's good to make a bubble trap, which is a series of partitions installed to stop the bubbles from making it back to the display.
I suggest drilling this tank before you fill it with water. As scary as it may sound, it's a simple step and will save you aggravation down the road.
This company- Aquarium overflow box 700gph
Offers you everything you need including the bit to drill and install your gravity overflow.