Bodywork repairs can be and usually are more expensive than mechanical repairs on a lot of cars and to fix the rust problems properly on your Fiesta will cost as much as you paid for it. Always a difficult choice on a car which was cheap to buy. It depends on how much you like your car and how long you intend to keep it. If the rest of the car is in really good condition and you can afford it I would get it done and that way you will have nothing to worry about long term.