[gentoo-user] Finalizing my backup system

[Grant]

I'm using backupninja to backup data from my laptop, desktop, and
remote server onto a remote desktop system.  backupninja is very
simple and is really just an interface to a few other programs
including rdiff-backup.  I'm not worried about a good restore method
for now, I want to focus on keeping it simple and protecting my data.
This is the first time I've set up a real backup system and I'd love
to get some advice from you guys.  I've got a few questions.

1. This is the first time I've used passwordless SSH keys.  root on
each system being backed up logs into the remote desktop as a normal
user to store the backups.  Is this pretty safe?  I suppose if root is
compromised on any of the three systems being backed up (via physical
access or otherwise), the remote desktop will also be compromised as a
normal user.  Maybe that normal user should be extraordinarily
unprivileged?

2. backupninja can email reports.  This works on my remote server
which runs postfix, but my laptop and desktop don't run an MTA.  Is
there a simple one that would be well-suited to a purpose like this,
or do I need full-blown postfix on my laptop and desktop?  Whatever I
choose, I'd also like to use it to send PORTAGE_ELOG messages from
those systems.

3. On each system I back up /etc, /home/user/backup,
/var/lib/portage/world, and /usr/src/linux/.config along with anything
special from that system.  Would anyone recommend I back up anything
else?  Some of the hidden directories in /home/user might come in
handy, but I think I can rebuild those without too much trouble.

4. I have 600GB of music and photos that I'd like to back up somehow,
but that is too much data to send to my remote desktop over my 20KB/s
upload.  How would you handle this?  I was thinking maybe two external
USB drives that I switch back and forth between being connected to the
desktop system and being stored in a fireproof/waterproof container
and hidden somewhere in my apartment to hopefully protect against
fire, flood, and theft.

5. Do I have enough redundancy with backups only being stored on one system?

6. Any ideas for backing up the remote desktop which is the system
where all the backups are stored?  I can't back it up to my desktop or
laptop because I'm behind some kind of a shared IP address.  I also
don't want to back it up to the remote server because that would
require SSH keys on the remote server and if the remote desktop is
compromised I don't want the remote server compromised along with it.

Thanks guys,
Grant