If you're really wanting discus then you should check out SimplyDiscus. That's pretty much the only fish they talk about over there.
As far as the research I've done goes, I can tell you that you will want to get six. They are a schooling fish and like to be surrounded by their own kind, but also need about ten gallons per discus as far as space goes (I think your 55 is close enough to 60). They like water between 82-86 degrees, so there aren't very many options for tankmates because that's pretty high. If you are growing out juvies then you need the tank to be bare bottom and they will need significant water changes (like 50% or above) at least every other day. If you get adults, bare bottom is still recommended, but not required as long as you keep your substrate clean and keep up with water changes. Adults probably don't need as many water changes but if you're keeping six in a 55 then I would plan on three a week at 75% or more. Discus really don't like nitrate levels above 5ppm.
Another option for discus would be to buy a proven breeding pair (emphasis on "proven") for the 55. They will probably need to be the only inhabitants, but will require a little less maintenance.
Water for the water changes will probably be best aged. You will want to get a container and put a heater in it to pre-heat the water, and a pump or air stone to help it gas off dissolved co2. This affects the ph and you want to keep the ph as steady as possible for discus. The actual ph value is not that important, as long as it's not something crazy high, but it is important that it's completely steady.
As for filtration, sponge filters, like what you have pictured above are highly recommended. Canisters are not recommended. HOB's are also good filters. Basically, anything that is a very effective filter, but is also very easy to clean and rinse off with each water change.
I don't actually have discus yet, but this is all the info I have gathered for when I get some discus sometime this year. I also have an account over on SimplyDiscus and they have a ton of knowledge for these fish over there.
One last thing, if you are serious about wanting to get into some discus, check out the video below. It is well worth the full hour and twenty minutes or so.