After using a few boxed solutions including several mentioned above (ClearOS, Untangle, Smoothwall, Zentyal, & pfSense) I ended up going with ClearOS. They all have their up's and downs, most of them have paid services that they will offer you which with a little configuring and time you can accomplish yourself.
From what I can tell your going to be interested in something more then just a simple router/firewall my suggestion is grab one of the free ones available build a low end machine with two nic's and have fun.
For some more reading take a look at this:
I settled with ClearOS because I wanted to have a full server at my disposal and it was lite on hardware requirements where as Zentyal in particular were heavy on server resources now while my home router is old it isn't by any means a slouch, dual processor Athlon MP 2200+, 2G Ram, 120G 7200rpm, (10 internets if you can guess the maker/model of the mobo /.) I have held that box at 80mbps inbound constant for days on end downloading..... "TPS reports" yeah that's it, without showing any marked reduction in its performance, since install its only been shutdown twice for hardware maintenance(upgrades) before I shut it down the last time the up-time counter showed 240day's.
After all of that I personally think Clear is more then likely something your client could easily use to monitor traffic to and from the network, utilizing some of the built-in features or adding in something like ntop for ultra detailed logs of everything going on anywhere on the network. Your client could easily access the the logs by going to an internal web page and reading the logs at his leisure, a fair warning though if you go the ntop route the amount of logging is immense when I said everything is logged I mean it you can easily have several gigs of logs per day if the network is heavily trafficked.