It depends on how crazy you want to go but its not hard. All you need is some sea salt,live sand,live rock to get the tank going. As far as filtration thats wear the tricky part comes into play. The key to a healthy salt water tank is water flow and filtration. Adding things like live sand and live rock give your tank the bacteria thats very important in a new tank. The method behind a protein skimmer is instead of you old fashion fish tank filter. A protein skimmer makes bubbles like 1,000,000 of bubbles that catch little bits of dirt that the naked human eye cant see. (you can see my protein skimmer in the last pictuer and the cut of dirt at top)The bubbles rise to the top of skimmer and pop inside a cup which collects dirt . You also want to have a powerhead which simply blows water and creates a ocean like current which is also important for filtration.
Mantis shrimp need to have good filtration because they are bad eaters( body parts all over the tank and they sometimes burry their food around the tank and dig it up to eat when food is low.

Just to let you know I had freash water tanks all my life and always thought salt water was so hard to do. I tried it and it was just as easy as any pet hobby as long as you do research you will win! ITS NOT HARD