Both my front arches we rusty, drivers side was worse than passengers, which I was told was salvagable. I replaced my drivers side wing and had the passenger side one ground back and re-painted. Needless to say I was back 12 months later having the passenger wing replaced. Once the rot sets in its very difficult to stop.
I used genuine BMW wings though I was very tempted to go aftermarket and fit carbon fibre replacements. I chickened out though as I wanted the car to look OEM and there is a general lack of info on the level of fit and finish you can get with carbon replacements. I'm not sure if there are pattern steel items but personally if you want the to last and fit well OEM is the only way to go.
CSL wings are no different to normal M3 wings, they are the same part number. If you are going to get them get them now, I have heard word that prices are about to jump as a result of the slump in the pound.
oh, and don't get me started on the rear arches
