Early S3's (APY - 210) had a slightly different (wider i think?) cambelt so it used to be 115,000 miles until you were advised to change them by audi, by the release of the Later (225 - BAM) engines audi were recomending 80,000
End of the day the S3 is a performance car and the belt would take a fair amount of abuse so i'd go for 60k / 4 years and get it done ahead of time just in case.
Good luck with your search
Mike