I would like to have the following backup system. What are your
recommendations (open source or closed source) ? Does this make any
sense at all ?

I want to backup every night my server's data onto DVDs. I have about
40-100 MB of new data per day. I have the following requirements:
- I want to reuse the DVD until it is full
- When the DVD is full, I want the tray to open and the backup process
to resume when I insert a new disc (probably the following morning).
- I want to do incremental backups in the following way:
* full backup every 4 or 8 weeks (e.g. every 4th sunday)
* backup from previous full backup every week (e.g. sunday)
* backup from previous day the remaining days. This way my maximum
incremental chain is 7.

Thanks for your time & help.