Thank you Dave and jchin for your replies, I will try to use them both to get it right. Actually this backup is meant for my daughter, and I think I will have problems making her use her NAS as the central point, even though I realize that is the easiest way to work it out (I use my own NAS for all my music, with backup (mirror right) to one other computer for extra safety).
The "delete question" was how to replicate a deletion correctly. The problem is if something is deleted by accident on a computer, and then from the backups as well - and how to make sure deliberately deleted files are not coming back again. This and overwriting a new file with an older with the same name are the biggest hazards with backups as far as I'm concerned. Dave's point about versioning is good, I already use it for my personal backups, and will do so for my daughter as well.
The "delete question" was how to replicate a deletion correctly. The problem is if something is deleted by accident on a computer, and then from the backups as well - and how to make sure deliberately deleted files are not coming back again. This and overwriting a new file with an older with the same name are the biggest hazards with backups as far as I'm concerned. Dave's point about versioning is good, I already use it for my personal backups, and will do so for my daughter as well.