If you think you can save money by buying the ballast, housing, reflector, & bulbs separately and are competent at wiring then go that route. Typically though you get the best deals just buying the Retro MH
& Retro VHO
kits available on the net.
bulb will cover about 2 ft so you need two for a 55 gal
tank. The wattage is going to depend on your corals.
Do you already have a canopy?