Ethan welcome to Reef Frontiers. You dont need a big skimmer for a tank of that sixe so you should be able to get lucky and buy one used from someone that is upgrading tank size. Here is a brief discription of the different types. Remember one thing its all about the bubbles no matter how they make them.
Airstone skimmer = simple and old design that uses an airstone for the creation of bubbles. Still a very effective system but the stones need to be changed out from time to time, also the cheapest.
Venturi= uses a pump to blow water into the skimmer and has a venturi valve that draws in air to mix with the water. Effective unit, the venturi will need to be cleaned from time to time. can be a bit noisy to.
Counter current skimmer= can be either the airstone or venturi type, the only difference is that the water enters from the top giving the skimmer water more contact time.
Beckett= Uses a pond fountain fixture for mixing air and water. Very effective unit but requires more of a pressure rated pump which can be pricy. The beckett nozzle has to be cleaned from time to time and can be a bit of a pain.
Downdraft= uses a strong pump to blow water over media such as bioballs to fractionate the water and air. Very effective unit but requires a strong pricy pump. no real maintence.
anyway that give you an idea on what and how they work
Mike