I would build.
You don't always get a normal drive when you buy a usb hdd. Sometimes you get a firmware restricted drive that won't work well if installed into a computer or worse, you get a drive that has the usb bridge build into it instead of the enclosure...
The other benefit of building your own is you get to control the quality of the parts. Grab a good enclosure and a WD Black, Red, or RE drive and you should be set for a long while as long as you dont mistreat it.