If you are buying a DVD player, dun need to crack your brain, most DVD players in the market are ok as long as you buy established brands from Japan or Korea.
Dun bother HDMI outputs unless you are a cable freak who like neat cables. The image quality from HDMI or Component makes no difference for DVD format as DVD resolution is standard TV resolution.
As for Blu-ray player, it's explained by neubie.
But adding to that, tf you have a HDTV, especially those that boast Full HD in 1080p resolution. It will be a waste without a Blu-Ray player or a game console like PS3 or Xbox360.......(like having an iphone without 3G internet)
Even the HD channel 5 from mediocorp is no match for the crystal clear content of a blu-ray disc.