Those freshwater kits are good deals, even if you are converting them to saltwater. My first tank was the 55 gallon kit from Walmart!
Did it come with a heater? If so, you will need that. The lights it comes with will be fine for now, if you have fish only. You will have to upgrade lights down the road if you plan on keeping photosynthetic animals such as corals, clams and anemones.