Hello Jonlee,
I guess it will depend on the actual model specification whether the car has a plastic cover over the sill or not - but you would be able to easily know that. Whatever cover it may or may not have over the sill, the sill will indeed be metal and is a structural part of the car. If the metal work has indeed been damaged then the extent of the damage would dictate whether the damage is professionally repaired or if a new panel would be required.
If it has to be replaced then unfortunately this is fairly major surgery since it involves welding to the A,B, and C posts as well as the inner sill and floor. The professionals may suggest a local weld repair instead, but without knowing the extent of the damage it would obviously be difficult to advise,
I would suggest you have a careful look at it and you will easily be able to tell if what you are seeing is plastic or metal. If plastic and its damaged then you could remove it to see what the situation is with the metal underneath, but my advice would be to take this along to a local trusted body shop and take their advice - that shouldn't cost you anything. While there, ask them to visually check the floor and suspension for you to satisfy yourself the damage is localised - rather than take chances with the unknown.
Perhaps you could let us know how you get on.
Good luck, Gareth.