since you got everything for so cheap, personally i would ditch the filter and gravel, especially gravel. filter you can probably clean by letting it pump bleach diluted water.
when i was cleaning out my 20G tank, i really didn't measure how many gallons to capfuls of bleach i used, i probably used a lot more than i should since the smell of chlorine was really strong. But it made the tank pretty sterile and after a ton of rinsing and scrubbing, letting it dry off, and a slightly higher dose of Prime in the water, pretty much all traces of bleach was gone.