Quote 2: "S'pose I went in with you guys. That's three hundred an' fifty I'd put in." The speaker is Candy. Why is this important to the...

You've gotten two very good answers to your question; I only have one other element to offer which might add to the discussion.

Lennie and George have a dream--a place of their own, a farm where Lennie can tend the rabbits.  It's just a dream, because they each have something (besides their obvious lack of money) which is keeping it from coming true.  Lennie would not be able to raise rabbits, as he is a compulsive "petter" and would eventually kill them.  His handicap here is his strength.  George's handicap is Lennie; he would forever be trying to keep Lennie safe and then trying to repair the damage when he failed. 

Candy is perfectly suited to join this venture.  He is unhappy and feels unproductive, so anything to look forward to with hope is a blessing to him.  We know, though, from seeing what happens to his dog, that Candy will not be part of the dream.  If he can't keep them from killing his dog, he certainly won't be able to assert himself in any meaningful way.  His handicap is not his missing hand; it is his inability to be more assertive and useful.  He lives in fear, and that fear will keep him from any kind of forward movement.

When Lennie dies, the dream dies.  George might still one day achieve his dream, but it seems unlikely since it was what George wanted for Lennie more than for himself.  It's certain Candy will never leave the ranch.  Hope is dashed and the dream, for them, has died. 

But that doesn't mean dreaming has no value.  It kept Lennie alive until the very moment he died.