I would plan on overfiltering your tank, so I'd say an AC300 + cannister (maybe fluval 203 or 304) since cichlids get decently beefy.
You basically have 2 ways to go in a 55...
1. African cichlids... you can probably stock the 55gal with 12-16 Lake Malawi mbunas and have a nice colorful tank. Lots of rocks and caves for hiding spots. There are a lot of things you can do if y ou go african! Lots of very colorful fish...and they don't get too large (for the most part)
2. New World cichlids... a 55 won't be able to hold the fish that get to 10-12" or larger for life. Blue acaras are nice, a female Green Terror... Jack Dempsey... male Convict. You should probably stay away from the very agressive cichlids. A few hiding spaces/caves would be how I'd decorate the tank.
Either route will allow you to use pretty much any substrate, I would check out some of the more coarse types of sand out there...check out some pool filter sand.
If you're thinking about live plants, the new world species will probably dig them out. And they will probably re-decrorate your tank how they see fit