Yes, you will need a submersible heater, not the freshwater hang on variety.
As evaporation occurs, you add freshwater. Salt does not evaporate. You will need a hydrometer to measure the salinity, aiming for 1.023-1.024. At times you will need to replace some saltwater, because salt tends to "creep" out of the water, getting on the rim of the tank and glass.
There is no reason to use a hang on or canister filter, as the use of mechanical filtration will increase phosphates and lead to algae growth. I would just use the skimmer. You will, however, want to add a small powerhead to the inside of the tank to help with water circulation, hopefully preventing detritus from accumulating.
You should take a minute to visit our reference library. There are some articles on filtration differences between freshwater and saltwater which you will find very helpful, and an easy read.
Water Quality Reference