Any day now I'm going to have to get the rear arches sorted. - Anyone know how much money I need to put aside given that the inner arches (i.e. around the strut tower) are what I'm worried about more than the external arch?
I just did this myself (replaced/rebuilt my RR inner tower/turret) and mine was REALLY bad. I had to remove the turret and then weld in patch panels where the turret itself was spot welded to, there was no good metal to even weld a new turret to.
I got a repair panel from Carz2uk, for about $50, but be forwarned, it is NOT the complete turret skin, it's only about the top 2/3rds of it and is designed to be butt welded to the bottom 1/3rd of your existing panel (usually they rust around the top of the turret, so hopefully you have something good to weld to). Also, the repair panels are for a 4/5 door sunny and don't quite line up right so there's a fair bit of metal work involved in getting them to fit right.
I've no idea what it would cost someone to have this done at a shop, but based on the work I did, I had roughly 25 hours of fabrication labor just in the metal work, not including and primer, paint or seam sealer work.