Is there an easy way of maintaining a saltwater fishtank? just how hard is it? expenses per month?

I have a 55 gallon freshwater tank right now with several convict cichlids, an eel, and an algae eater. I have a good filter (dual filtration system), one power head (have 1 or 2 more laying around), an external thermometer, air pump, vacuum, buckets, scrubber, basically everything for freshwater. what else would I need for a basic saltwater aquarium? are there any shortcuts i can take to make the existing freshwater into saltwater or do i have to start from scratch? also what kind of saltwater fish is "hardy" and good to start with for a beginner in saltwater?