I don't think this CPU has Speedstep, so I could see the laptop using maybe as high as 35W-AC at idle. That is worst case, though. Could be as good as 20W. My 1.7GHz Pentium M draws 12W with the screen off, but Speedstep makes a big difference.
An external 3.5" drive is going to run you 10-15W-AC. Going with an external 2.5" drive you'd probably be down to 5W-AC. Going with a 3.5" drive like the WD GreenPower that automatically unloads the heads very aggressively could also get you down close the 5W-AC mark and get you lots of GB/$.
So depending on configuration, I think you are looking at a draw of anywhere from 25 to 50W-AC. My guess would be 35W, especially if you go with an energy efficient external drive. Even 50W is pretty good for a modern desktop build, so I'd say this is a good use of what you've got lying around. A couple of notes:
* I wouldn't expect the laptop to survive more than 2 years of this use case because it is basically the opposite of the designed function.
* Look into revoSleep and similar utilities to potentially shut down the external drive when not in use. In a 24/7 situation, saving that 5W when idle, via a free utility, is a nice win.