Most DVD players with USB do not support external hard drives—only USB flash drives. If the primary need is for a DVD player and the ability to play digital files is incidental, then go for a DVD player like the Philips DVP3568 (4,500).
However, if the primary requirement is a device to play digital files from hard drives, get an HD media player like the Seagate FreeAgent Theatre, WD TV Live or Iomega ScreenPlay HD. As regards your second query, a home theatre system is designed to replicate the surround effects that you get while watching a movie in a theatre.
It usually has 5 (or more) speakers and a subwoofer. The system works best when a movie is played from a DVD or Blu-Ray disc with built in 5.1 channel digital sound. A home theatre system can play music too, but usually only two of the five speakers operate in music mode.
Similarly, a stereo music system (with two channels, and only two speakers) can be used while watching movies, but without the surround effects .
Note that some 2-speaker systems and soundbars can create faux surround effects by bouncing sound off the walls.