If there rust extends all the way into the wheel well as it seems to in the picture, don't bother if you'd consider this a shitbox.
If you're really up for it, looks like you'll have to re-do the whole sill. And if you discover more rust when opening it up, probably a part of the floorpan.
Like I said, consider letting this one go. Especially if it's the same on the other side and it's a shitbox.