No, the second isn't grammatically correct and it doesn't make sense. The first one is grammatically correct, but it has an awkward mixture of the active and passive voices. I would suggest: Let's hope that we don't just end up buying sneakers with the money that we save on fuel. OR Let's hope that we don't just end up using the money that we save on fuel to buy sneakers.

1

One little word, "in", will fix #2: Let's just hope that the money we save on fuel won't end up IN spending on new sneakers.

Option one is correct

The others are correct. #2 sounds like the money will be doing the action of spending, which is impossible.