My question in response would be; why is your web server using 30Gb of data? I'm assuming that your data (web files, database etc) does not weigh in at 30Gb - lestwise you would be wanting to back that up. I would say, trim your web server to what are its bare essentials.
Otherwise; you hit the usual suspects. You should be backing up lighttpd's config files, php's ini files and mysql's config files in addition to your data. I would also recommend backing up the log files from the system (if you're not sending them to a centralized syslog server) to aid in diagnosing what went wrong with the original system in case of failure.
If your user configuration is sufficiently complex (more than just the usual mysql, php and lighttpd users - I would also suggest looking into a federated system (LDAP, OpenDirectory or Windows Domains depending on what OS your server is running) to facilitate quick fail-over to another server.