Intrusion Detection System for WordPress sites
You can't install a true IDS on shared hosting, this is the host's responsibility.
An hack-ish solution:
You could create a script that ran periodically (using cron or some other mechanism), that would checksum all files, and compare the checksums with a previously stored record, then notify you if there are differences.
To find out if your script itself was deleted by the attack (1), you must also create a script sitting on a remote server (something like Google App Engine, perhaps), that pings your shared-server-script, and checks if it gets an expected result (a hash based on given time, perhaps) – if not, it emails you.
(1) This is actually quite unlikely, most attacks don't delete files
http://www.la-samhna.de/samhain/
However this won't work on shared hosting, so you'll need either a vps or a dedicated server