Life is unpredictable. For instance, he wouldn't have guessed he would win the lottery.
Life is unpredictable. For instance, he couldn't have guessed he could win the lottery.
I'm a little confused. Both sound good to me (with a slight preference to would).
...wouldn't/couldn't have guessed...
Using "wouldn't have guessed" gives the meaning that it would be unexpected (an improbability) for him to guess that he would win the lottery.
Using "couldn't have guessed" gives the meaning that it would be impossible (an inability) for him to guess that he would win the lottery.
I would choose couldn't in this sentence because the intended meaning is not so much that he was unlikely to guess, but that he was unable to guess (or, more accurately, know) that he would win the lottery.
...would/could win the lottery.
Using "would win the lottery" means that he, in fact, went on to win the lottery. In other words, would is used in this context to express something either happening or not happening.
Using "could win the lottery" means that it is possible for him to win the lottery, but does not tell whether he did or didn't win.
Of these two, I would choose would in this sentence because the intended meaning is not that he had no idea it was possible for him to win the lottery, but that he had no idea it was going to happen.
Life is unpredictable. For instance, he couldn't have guessed he would win the lottery.
I would probably word it, "Life is unpredictable. For instance, he couldn't have guessed he would win the lottery." Why? Because "could/couldn't" means he was or was not able to guess or win, whereas "would/woudln't" means he did or did not guess or win. Essentially:
"Earlier on, he was not able to guess that he, before now, did win the lottery."
That being said, native speakers don't usually make too big a deal out of could/would in situations like this...though in this particular situation, there is a difference between whether you can win a lottery and whether you do win the lottery.
