So my question is, can I administer the firewall from the internet/WAN side of the firewall at all?
Sure you can.
Also what would be the best/safest way of setting up M0n0wall to access the NAS.
m0n0wall doesn't
access your NAS, just directing traffic to and from it.
I’ve got DYNDNS working as I don’t have a fixed IP address, I won’t always be connecting in using the same IP from the web, am I better to NAT the IP and random port number through to the NAS?
Not sure why you want to do that or maybe I don't understand what you mean.
m0n0wall comes with a DynDNS client.
What kind of services will you use over the internet?