perhaps i'd start early on (chronologically), the hummel etudes are a pretty good bet , pretty too, and are thought to have heavily influenced the more popular later chopin works and others. they're neglected all to often as most of hummels fine piano music is unfortunately.
edit-sorry i'll clarify i do not know if they are appropriate for 'intermediate' level students, honestly i can never 'grade' where people are i just know if music is, not hard, sorta hard, very hard, impossible yeah right no way in hell right now hard.
some might be doable by an early talented student with careful instruction and lots of dedicated smart practice.
to my knowledge i think there are a couple of sets , the 10 etudes and the 24 etdues? i can't find a complete set of 24 all the scores in public domain seem to be missing nos 18-24
attached is what i have