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.
Also Jupiter as a fueling station for the hydrogen..
If you want more mass and less water, you can use Ganymede, which also has a healthy magnetic field which suggests it would help enrich the core of any post-collision planet.
Also, trying to orbit one planet or large moon around another is going to be very difficult, which is why few scientists believe that Earth’s Moon was captured. There is no way to slow it down and establish a stable orbit. Crashing two large bodies together into a stable orbit is a lot easier. As for moving planets closer to Earth’s orbit, remember that they will influence Earth’s orbit so that’s a dangerous game to be playing.