I second sand as a substrate. The gravel you can buy at Home Depot is cheap but as was pointed out is often not inert and doesn't really look that great, IMO. Sand looks fantastic, though, and is really essential if you've got bottom dwellers that like to burrow or have sensitive barbels (or both) such as corydoras catfish or just about any type of loach. One 50lb bag of play sand from Home Depot should cover you. Just be sure to rinse it well before adding it to the tank.
What type of filter is the one that already came with the tank? I recommend getting the Emperor 400 and running it alongside the filter that came with the tank. Also, grab yourself an empty 10g tank from Petsmart (which should run you $11-12), a glass canopy for it, and a 50W adjustable heater. This eats into your cost a little, but you can definitely cut some corners in the decor department and instead invest in a quarantine/hospital tank. You can leave it empty until you need it. Any time a fish is sick (or you add new fish, which you'll definitely be doing) just fill it up, add water conditioner and the heater, and move the extra filter from the 55g to the 10g and there you go - instant quarantine tank. You'll be glad you had it available.
The 300W heater isn't quite as effective as two 150W heaters, but so long as it works it should still get the job done. Just be sure to get a glass thermometer for the tank, and set the heater in horizontally. Place the heater in a high-flow area for even heat distribution.
As for other decor, you can get a big piece of driftwood from the LFS as your centerpiece. Then, you can make plenty of hiding spots and caves by just stacking up some shale or slate from a landscaping center. Usually fish stores will sell rocks, but they often charge $2-3 a pound wheras the same rock at a landscaping place is $0.10-0.20 a pound.
Can't wait to see the tank full of fish!