Well I'll list this anyways...
1. Tech wouldn't be available until like 2300 to do so effectively, if that, that's very generous.
2. The gross capital generated in this entire RP wouldn't be able to be enough, and if we were to make it enough (slavery), all nations would have to contribute, and ¼th of the world's population would die of starvation, malnutrition, or other nourishment-deprivation-based diseases, and the Terran global society would fall into disrepair.
3. You would have to either nucleosynthesize all the oceans and atmosphere or find a lot of it from other sources, as well as purify it where found.
4. You would have to ship these supplies to Mars, and have a space-capable fleet to do this.
5. You would have to mend the electromagnetic field of Mars, otherwise you'd be subject to radiation, and frequent meteor showers.
6. With unsuitable gravity, the atmosphere would escape into space, and your oceans would both freeze and evaporate. This would have to be modified.