I gather this is a common problem. I have been told i need inner and outer rear wheel arch panels both sides. I see these advertised a  a range of prices. What do i need and is for the best. Car in 2005 Subaru Impreza saloon STI wide track type UK 2.0L.  cd

Beware cheap jobs. they rust from the inside out so it's 99% probable the damage will be considerably worse on the inner arch. untill the external panels are stripped off to see how bad the damage is you wont know. it can affect everything from the fuel filler to the suspention tower.

Im not trying to cause you panic as it is unlikley not repairable, I just warn you as a paint over it cheap job will only make things worse in the long run. Realisticly your prob talking anywhere from £800 a side upwards for a decent job including blending in etc.