The rear arches are less prone to rust usually on these!
Reason for new rather than used, bodyshop said there are two options.
1: Cut out the affected section, and weld a new section on, flatten it, prime, paint and protect. Usually this would be fine, but long term might not always be the best option.
2: Replace the whole wing with a new one. Obviously much better for long term.
A used wing in good condition would also be fine, but you can never be sure there isn't the tiniest of holes in the arch, even if microoscopic (as is the case on mine right now) given that the used wing would also be at least 10 years old.
So I'd rather go option 2 and not have that used wing risk looming over.