Welcome to the biggest problem & world of expensive pain for every single Mk1 Clio owner
Your body shop guy knows the score, impossible to quote on these until you know how deep the rust is and more often than not, it goes much deeper than what you can initially see and you can only really see once you start digging away.
It's because of "sand down & respray" jobs done in the past that they need doing again - that's basically a bodge on these. The only proper fix is to chop out the rot & replace with fresh metal, if it's been done in the past and needs doing again you can be assured it is that bad and most probably a hell of a lot worse than what you can see
Arches are also a problem as the flared ones for the 16v & Williams simply do not exist, the only option is to source them for a lower spec Mk1 Clio and they then have to be massaged to fit correctly - the panels for those are readily available from
https://www.carz2.co.uk/renault-clio...re-96704-p.asp