Current research is considering the possibility of using large lava tubes as living space (and probably greenhouses). Shelter from stray debris strikes and poor weather, insulation... They have proven water is obtainable on the planet's surface. Soil is easy to manufacture with a few basic starting pieces and patience. The biggest issue would be light. Solar radiation is less than half of Earth, an efficient power system to run lights and heat would be essential.
You can't put the rest of the atmosphere back on Mars with terraforming, without constant replenishment (think plate tectonics) it would just drift away again.
