I'd say the goal is to find a survivable planet that can support billions of people. In that sense, the amount of water ice that is in a couple of permanently shadowed craters is really negligible. And on Mercury you're talking about only a narrow zone around the poles that you can support people if you excavate massive amounts of rock underground.

If you're talking about the energy required on Mars to change the atmosphere on a planetary scale, is there a source that says that would be a limiting factor? None of the research on this that I've seen considers it to be an issue. Mars receives about 1/3 the incident sunlight that Earth does, measured at vacuum. But the amount of that incident sunlight that will reach the surface will be higher on Mars due to the thin atmosphere and lack of cloud cover.