Confession time: I haven't read Watchmen, Dark Knight Returns or Sandman... I'm wondering which of these are best to try out first.
Definitely Sandman.
Then yeah, just start with the whole Mobile Suit Gundam: The Origin. It's a retelling of the original anime with a bunch of added context to the characters' histories.
Wait, what? Can I answer this question with "No"?
Is it a better overall telling of the original Gundam story?
And I've already got one ongoing giant robot fiction infecting my life, I don't need another.