Your best bet, if you're considering a 55, might be to consider raising a large chichlid, like an oscar, along with a pleco. For this, Penguin makes a real nice power filter, as does Tetra.
If you want to raise smaller fish, I'd strongly advise against livebearers, like mollies and guppies, unless you want a kazillion fish. Consider angelfish, tetras such as neons, cardinals, or perhaps tigers, barbs, maybe a male betta. Good scavengers for this set up are corydoras of any variety.
For this setup, an undergravel filter, powered by two large pumps, is adequate.
You will, of course need a lighted hood, background, live or artificial plants, heater, and I'd spend the bucks for a Python system to make cleaning it/changing part of the water on a bi weekly basis easier.
If you don't have well water, you will need some chlorine remover/conditioner. In case of the dreaded disease Ich, remedy in either tablet or liquid form.
Find a small, locally owned pet shop to get your supplies, if possible. This will cost you a bit more, but you will get personalized service from people who really CARE, and who will want you back, often. They'll know about your local situation, and can advise you, as well as providing quality fish at reasonable prices.
Have at it, and good luck!