You mean weld in a flat piece where the filler cap is? And then paint the panel to match the rest of the car. You don't like the look of the filler cap I assume?
Is there some rust in that rear fender?
Honestly body work and paint is very expensive. Your car looks shiney and very presentable right now.
You could very easily log roll into an entire paint job. And I'll bet that will be $5K to $8K.
What specific interior things do you want done?