1. Do the digital receivers all function the same way or are there
different features/functions to be aware of? Any receiver brands
that are clearly better than others?

When you say "digital receiver", I can't help but think of an audio
component. You mean a "TV set", I assume.

My understanding of the term is that a "digital receiver" is a
digital TV *tuner*. It doesn't (or at least might not) include a
monitor. It takes a video signal (ATSC, cable, or satellite, or
some combination of them) and converts it into something a TV set
or DVR can accept (S-Video, component video, DVI, HDMI, etc.) It
might be able to downconvert (so an old analog TV set can display
HD signals, although with less resolution) or upconvert.

I would not expect an audio digital receiver to include speakers.