Go with what's already setup, if ssh on the work box is already an open/monitored/supported/audited service then try to do the rsync via that. Not opening up new ports/services is generally safest. Not opening up insecure protocols to the internet is even better =)
You can get ssh access to the ReadyNas (if you don't mind some hassle from Netgear in the event of a "i deleted my nas" support call). Then
rsync -e ssh from the command line which leaves nothing else to setup. Auth, wire security and user/file permissions are all provided by ssh/remote shell setup on work box.
For opening up ssh on your home network:
The dynamic IP hassles can be covered by running a free dynamic DNS service.
Restrict access to the port to your work box's public IP.
Some routers allow you to set a source IP in the NAT rule.
SSH can be secured with
iptables and more
As DarkHeart mentioned, rsync by itself over an unsecure network is not a good idea. The tunnel and vpn mentioned is a good work around. You can keep the aforementioned ssh tunnel up with autossh. You may want to depend on some rsync security if your going to leave a tunnel up depending on who else has access to either end.
Also, if you haven't already, discuss what your doing with someone. Detail what data is going in/out of the network. Think about what data might be able to go out if everything goes wrong. Document your process somewhere.