I think that if Jupiter were a good star candidate, it would have become one. On the other hand, Jupiter has a few large water-rich moons to play around with so it would be possible, for example, to hit Mars with Europa and still use Callisto and/or Ganymede if another hit is desired or to hit Venus. Sure, comets could be used to add water to Mars but one of those big moons would add a lot of water and, as a bonus, re-liquify the planet and get a magnetic field going again and possibly add a nice moon. The evidence strongly suggests the magnetic field is very important for life and if it’s rare to keep one for a few billion years like Earth has, that’s a real strike against the odds of extraterrestrial life and especially intelligent extraterrestrial life. The magnetic field keeps the solar and cosmic radiation out and also keeps the atmosphere from being whittled away by the solar wind.
My idea for trying to terraform Venus (if it isn't just easily to pick up and move elsewhere - even onto a space station or generation ship) would be to try to precipitate all that sulfuric acid floating around in the atmosphere first.
I actually had a 'move Europa' idea like QA, but mine was to just move it into orbit around Mars, not crash the moon into the planet (doesn't Mars have enough water as it is? Europa would add mass, and thus a closer gravity to Earth, but might lead to a water world if the two were smooshed together - I don't know the size differential between the two, but Europa is one of Jupiter's largest moons - update: Europa's mass, .008 Earths; Mars' mass, .107 Earths - Novo Martians would be tall and weak). If planned correctly, could Europa be used to tug Mars closer to the Sun (and warmer climes)? I'm guessing it wouldn't be enough to kick start Mars magnetic field. Now-waterworld Europa could then be used for aquaculture to supply colonist ships heading further out into the Solar System, or for food to supply to space stations made from the Asteroid Belt.
Just my daydreaming.