I second what BASSNMAN just said. One Penguin 330 should be enough although you can add a second one later if you wish. If you were going to populate your tank with messier fish like large cichlids or goldfish then the second filter would be definitely recommended. If you add live plants that will be even better.
Also, when you do add plants then you will want to eliminate, or greatly reduce, the use of carbon in your filters so that you don't filter out any nutrients in the water.
As far as heaters go, a good rule of thumb is to provide 5 watts per gallon. So for 55 gallons you should have at least 275 watts. This is best distributed between two heaters so that in case one fails you still have one that works. You should put them on opposite ends of the aquarium for more even heat distribution.
Gene Heitman - 12 tanks (11 freshwater and 1 saltwater), 206 gallons, 20+ species of fish/shrimp/snails, 52+ species of plants ... 10 years ago I just wanted 1 tank with some fish