I can only comment on the Foundation trilogy: They're an amazing piece of literary and SF work. Each book (Foundation, Foundation and Empire, Second Foundation) is self-sufficient, to a degree (i.e. you get sufficient 'recap exposition' to follow the story). A complete collection, though, is a better investment - you will get MUCH more enjoyment out of it.
Note that Asimov also wrote other novels / books / short stories in the same continuity, which are NOT included in the collection you referenced:
Foundation's Edge and
Foundation and Earth are sequels and true fans of the original series agree
that they do not exist. They are completely different from the political thriller / cog in the machine / unstoppable glacier of history books of the original series.
Prelude to Foundation and
Forward the Foundation are prequels and they are...meh - at least they do reference some of the political / historical science you find in the original series. I suggest you get them AFTER you read the series, if you like it, just to see how some of the characters came to be. Note that they link the 'Foundation' universe to the 'Robots' universe.
And, finally, there's the 'Empire' trilogy (
"Pebble in the Sky",
"The Stars, Like Dust" and "The Currents of Space"), which discuss the rise of the Galactic Empire - the collapse of which triggers the 'Foundation' trilogy. Essentially, a prequel series, long in the past of the Foundation - timeframes are comparable to SW:KOTOR and SW:OT.
Also,
this audio adaptation is GOOD, if quite old-fashined and a bit campy.