I would leave it for the bodyshop to do the whole lot, as long as they going to strip it right back and fix any impending rust rather than just what is showing.
All my arches are showing sighs, but the rears, particularly the passenger side, is quite bad, I put some hammerite rust stop on it as a temporary measure, but in a month or two I will be sending off to the shop. My mechanic has a guy that is doing me a trade price of £400 for the lot, and that includes painting the whole panel again as the metallic is apparently hard to colour match.