Ok, well a test kit is a good start. You need a liquid one that tests for ammonia, nitrite, nitrate and pH.
Your tank should be cycled after six months but, as you say, the meds may have knocked it back. You'll know when the filter is cycled because the ammonia and nitrite will remain at zero without needing to make water changes. The nitrate will also steadily increase over the week.
The fins should grow back if the meaty part where they attach to the fish isn't severely damaged. The lips may grow back depending on how severe they are damaged and assuming the rot is stopped now. He'll eat fine as long as he doesn't have to nibble the food (obviously), feed small flake, something soft.
A water change will always help because it dilutes any toxins that are in the water but you need to add an appropriate amount of medication in with the new water to keep the med dosing stable.
You shouldn't need to steralise anything, just make sure the tank is well maintained. Clean the substrate when you change water and clean your filter sponges in tank water (not too roughly). Clean any collected debris from ornaments and plants. remove any rotting stuff like plant leaves.