Repainting re-arch due to rust

Hello All,

I’ve noticed that my 2010 met-blue Ford Focus is beginning to show signs of rust on the nearside rear arch. The other three are fine.
I think it’s that arch which is a because if you look really carefully you can pick up signs that it’s been repaired before professionally, followed by a mediocre job of painting the rust in the wrong colour blue.

Pictures below.

As the car is low mileage and only eight years old (and new to me) I am keen to find a solution for the problem which will hopefully last for another 8 years.
Can anyone give me an indication of the price I am looking for a repair?

I’d also be interested in hearing about the process I should expect for a repair – if they take it back to bare metal is that enough, or does it need a rust treatment too?