You can use the AC's, but you won't want to have any media in there, just have them pushing water. The primary biological filtration is live rock (you'll want about 70-80 pounds) and live sand. The bacteria colonize the rock and sand, and the depths of the sand provide an area with low oxygen levels for the nitrate processing bacteria to thrive. This is also why you want the fine grained sand--a pound of small grain sand has way more surface area than a pound of river rocks. The more surface area, the greater the potential colonies. In conjunction with this system you must have some critters to maintain the sand bed and clean up solid waste--hermit crabs, serpent stars, snails, ect.
I never recommend anemones (the 'living plants') for a beginner. They're needs are not completely understood, and most live less than 2 years in the aquarium--heck, most don't make it to 6 months. For something that can live for hundreds of years in the ocean, it makes no sense to kill it off in an aquarium. But, clowns are perfectly happy without them, and the captive bred fish might not take to an anemone anyway. Clown breeders don't keep anemones in their tank, which should prove that the anemone isn't needed. If you later choose to purchase one, I recommend you have your tank firmly matured--6-12 months down the line, and research carefully to make sure you are meeting it's light requirements and can feed it enough. Keep in mind that not all anemones are acceptable hosts for all clowns.
A clown will be fine in a 55. There are many fish that you can have, but you'll need to look around and see what interests you. Tangs aren't a good choice--they tend to be very stressed and prone to ailments in smaller tanks. However, there are lots of gobies, basslets, wrasses, pygmy angels and cardinals that would work great.
There are many different powers of powerheads. Ones used in FW tend to be on the smaller side, so don't create the same amount of flow. However, it sounds like you might have seen a truly specialized setup, designed to mimic the water currents. The powerheads for these are very large, and are controlled by a panel that turns different ones on and off to create a wave like motion within the tank. They can be costly, and while cool, I haven't seen any proof that they benefit the tank enough to justify the expense. Strategically placed powerheads will work fine.