There is not really much more maintenance per se. The larger the tank, the more expensive the setup will be but it is also easier to keep stable. The main difference is monitoring salinity levels (topping off with fresh water as it evaporates) and that you have to pre-mix the saltwater for water changes. This will require a bucket or container of some kind, an additional powerhead to mix it, and a heater.
A 56g is a nice size, and you won't be too limited with fish choices. Most SW fish prefer not to have others of the same type so research is important. A lot of fish also get too big and/or aggressive for "normal" tanks.
Liveaquaria.com has a good list of beginner fish. You'd want to stick to the peaceful ones. Ocellaris or Percula clowns, royal gramma, various gobies, a blenny, firefish, and various others are good starter fish. Personally I love a neon blue goby - tank raised, very hardy and peaceful, come up and sit on your finger. Great little guys.
The filter probably won't be much good on the SW tank, a good hang-on-back protein skimmer would be a better investment. The live rock in a saltwater tank is the primary filtration, plus powerheads for water movement and a skimmer to remove the particles and nutrients. You can do without the skimmer, but really a good quality hang on back skimmer is a good investment and is not any harder than a hang on back power filter - you just empty the cup every couple of days, and give it a good wipe down.
If you want to use the filter anyways, it won't hurt anything for just fish and snails and such. The simplest setup would probably be your tank with light, powerhead for water movement (or 2 powerheads - I like hydor koralias as they are super quiet and have a nice magnetic suction cup to keep them in place), and live rock. With or without the filter or skimmer. You can start with mostly dry rock and just a few pieces of live, and it will all become live eventually.
Resist the temptation to get fish right away. The live rock is called such because of the bacteria that colonize it, and as some things on the live rock die and the bacteria multiply, the tank will cycle. During this time, don't have any fish in the tank (or other critters like snails, shrimp, etc). Read up on cycling a saltwater tank for more info about this, and use that time to get a good idea about what fish you want to get.
