At first, always get a unit with its own PSU.
As for the DIY vs. "buy ready-to-run", in former days I might have voted for DIY, especially because of the price. Nowadays I am not so sure. The main advantage of a ready-to-run drive (from a known drive maker!, not from 3rd party like LaCie, Verbatim, whoever) is the absence of compatibility problems. My personal favourite is LG, but YMMV.
That is USB. 2.0 HiSpeed.
If you want eSata or USB 3.0, then you have to join an internal drive with an enclosure yourself.