The Carl Sagan proposal won't work. Even if seeding worked, the carbon and oxygen would get back together. Ultimately, it would take very high technology and, by present standards, immense transfers of energy and materials, probably with permanent artificial structures to maintain Venus (sunshades and reflectors for a reasonable day/night cycle). It's physically possible, but it wouldn't be easy. There are also likely limits to how quickly it could be done even assuming very advanced technology and the easy transfer of great amounts of mass around the solar system.

If we want to try terraforming, I think Mars would be the first choice, but that would require high technology, and probably permanent artificial structures as well. The requirement for self replicating (and self repairing) machines is pretty much a given. Those replicators could be macro, micro, or nanoscale.

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