Hans asked whether Replicator warns about not enough free space before beginning. It does not.
I asked whether he prefers to know if a replication fails after it's been attempted, which means it backed up everything it could, or:
My preference would be Replicator shows a warning in case of insufficient space. Then it's up to the user to proceed or not.
The entire contents of the Source tree would need to be checked against the contents of the Destination if the Job is configured to only copy files that have been updated. This information would also be useful for providing a progress bar (there's a wishlist item here for Replicator to display better feedback during each job).
Most high end backup solutions I used in the past would always create a file list before beginning a backup. For incremental backups, that data would be needed to be checked against the job database. Creating the list of source files was quick compared to copying all the data to tape. :)