The arches will be fine if you clean out the dirt with a hose and if they havent been repaired after an accident.
My mums clio is W reg, coming up to 8 years old now(had from new) and there isnt any rust at all on the arches, due to me cleaning them out once in a while/and its never been repaired at all. Any rusty ones you see will be due to bodged repairs mainly.
Wasn't there a design flaw with the williams/16v that helped them rust, normal mk1 clio's always seemed to be less rusty.