Think Welles' version would've been great?
I can't help but wonder how that would have turned out. His Radio Theater version of the book which this doc uses throughout does give us some insight though... It's still available and there's even a copy on youtube. It was lifted of a record so the sound quality is terrible. What's much worse is the length. He had to truncate the book into a 35 minutes show and the story suffers a lot. A lot. Kurtz (played by Welles) is barely in it for more than a minute or two. The atmosphere works quite well, however.
Note: Welles radio version is closer to the book and Tim Roth/John Malkovich Heart of Darkness adaptation for TV from 1993.