It would have looked amazing - the visuals bounce off the page, starting with the image of a skeleton underwater. The setting of a fictitious South American country is an interesting one. I'm sure the acting would have been fine.
But it's a dull story, despite death and greed. I can't imagine why Lean wanted to make this. I'm not surprised Steven Spielberg bailed on it. I wasn't sure why we were meant to care. Or what we were meant to feel other than admiration of the photography and production design.
The story involves a British man wanting to work his father's silver mine. There's an Italian Nostromo who is meant to be awesome, I think, then becomes a bit corrupted. There's a dodgy doctor, some dodgy generals. The male characters aren't very interesting or believable but at least they're more active than the female characters. I simply can't see what Lean saw in this unless it was the novelty of the South American setting.