I'm looking to have a few worlds that were relatively easy to terraform as a first wave of colonies.

The first colonies in this setting were meant to be on worlds that were relatively easy to terraform with only one planet already able to sustain life. These easily terraformed planets were meant to be lifeless when discovered, no plants or significant microbial life. They were meant to take 20-50 years each to terraform, some being faster than others.

Is it possible for a planet to have an atmosphere that cannot support life that only needs one key addition (oxygen, water, etc.) to convert a world for human habitation in that 20-50 year span?

What types of planets, if any, would I be looking at that only need something that mankind with technology slightly ahead of our own could affect in such a short time span? Most of their advancements are in propulsion, limited FTL capability (requires gates on both ends) and energy production.

Are there any planets that meet these conditions in our own solar system that would be good testing grounds for these processes?

And what would the conditions of these planets likely be after terraforming?