It looks like setting up the firewall rules are going to be the trickiest thing here, using the iptables commands. Since I'm replacing an existing router with several existing rules, and I don't want the web/mail server to be down any longer than I have to, I'm going to take some time and learn this stuff before I try swapping things out.
I've seen a number of sample scripts in the OpenWRT forums, so I don't think this should be too terribly difficult once I get to that point. One thing that I'm noticing a lot of is that the commands get repeated over and over for each port/server being processed. Once I get my rules working, it would seem that setting these up in a loop would end up being a lot cleaner to maintain, just add a new server address and the desired ports, and let the code loop through all the dirty work.