Sorry, late for the LAN Party.
So, you're basically talking NAS (Network Attached Storage), which is a fancy word for file server.
You might add Gigabit Ether in the equation and gain speed: there are many cards supported under 4.x. There's some inexpensive hardware out there for Giga, but I've not checked it against the FBSD HCL. 4.9 RC2 LINT file shows entries for adapters based on the Broadcom BCM570x, the Intel Pro/1000 family, LXT1001 Netcellerator chipset, National Semi DP83820/21 and many others, probably 50 or more models in all...heh, now I read that you're already considering this. You've checked prices, I imagine, so I won't, yet.
About the only alternative I can think of is to use RAID, and trust that you're not gonna lose more than one disk at a time. Vinum is likely an acceptable substitute for actual RAID hardware (but I'm a tad sketchy on that as well...back to the Handbook...)
Of course, FBSD will run on ancient hardware, so you wouldn't necessarily have to spend a fortune to put a dedicated backup box together, but if you run GigaEther, you'll probably need a modern ATA bus to keep up, so best figure $350 or more just for hw, not including the drives or Gigabit Ethernet.
As for the woL, that's a decent possibility. I backup sites via php and cron over the WAN, and the times are pretty consistent.
I assume the NAS box will be in another location (catastrophe protection?)
I guess a good question would be the actual mechanics of the thing... Dump via NFS? DD via the same? Certainly couldn't use tar ...