So I believe the Mini would consume, on average under use, about 55W, which is a rather commendable figure for an official desktop and not a nettop. Adding peripherals will obviously effect this, a portable hard drive can pull up to 18W (maxing out the USB power supply). I'd definitely say opting for a bigger HDD on purchase is better than getting an external drive.
I do have a question for Mac Aficionado's, is the OS X Server akin to Windows Server 2008 or the Ultimate editions of windows? I was wondering if the Mini 1TB with OS X Server is as usable and accessible to a new user as any standard OS X (I'm rarely a Mac user, but my next purchase is likely to be Mac, so I definitely don't want to get my ass bitten by either losing out or screwing myself over by lack of knowledge when making a purchase).