The Reefpack 200 would be a great start for a 25 gallon tank. It will get your protein skimming and mechanical (filter floss) and chemical (carbon, phosphate remover, or whatever else you want to use) filtration taken care of. It comes with the filter floss and a filter bag that you can fill with the media of your choice.
Afterwards I'd consider an auto top-off as your next purchase. It will make your life a lot easier, and, especially on a smaller tank, the more stable salinity will help your corals do better.