This will probably come in a little higher cost than you'd like, but consider getting a Synology NAS box (http://www.synology.com/us/index.php), and a pair of notebook drives. I've had a home server of one sort or another for years. Back in the day I had top of the line multiprocessor Compaq server with a full RAID array. These days I live completely off-grid. The power draw of that beast would crush me.
Seeking a better solution I picked up a 207+, and then modified the brackets to take a pair of notebook drives. I measured it at 12 watts. It doesn't have much processing power, but I run fetchmail, dovecot, slimserver, and of course file sharing services on it. It would easily run a web server, though I host my website externally. I have mine configured with cron to shut down in the at night. Newer versions have automatic support for starting up and shutting down based on the time. If you're not hosting a web site, you can save more energy and money.
For me, running a server without RAID or mirroring isn't an option. That's one of the reasons I chose this solution. I use a USB harddisk for occassional backups. I've never hooked a USB CD-ROM to it, though I don't see any reason you couldn't. It's running Linux under the covers, and it's relatively easy to cross-compile software for it, or simply use optware (http://www.nslu2-linux.org/wiki/Optware/Packages?from=Unslung.Packages) to get what you need.