De: "I'd like to know the reason for choosing Mars."
I think the short answer is because it is the best PLANET, and it is resource rich, specifically including BOTH CO2 and H2O. The combination allows O2 + CH4 (methane) to be synthesized, with sunlight, thus allowing rocket fuel, and other vital, longer-chain organics, to be manufactured near-trivially, in limitless quantities. (FWIW, Musk's rockets burn CH4 directly - which is a major departure from prior rockets! That makes it possible for Space-X to manufacture its own fuel and for the fuel to be "clean", sustainable, and climate neutral)
But I suspect the longer answer includes: "to get away from earth". Only by getting far enough from earth can the umbilical cord be cut for giving up and returning. And I also think Mars is far enough away it would not likely be included in an earth-based nuclear war or other catastrophe.
But here is what Elon says and what we know:
http://www.inverse.com/article/33116-elon-musk-why-mars
[Venus is a super-high pressure ACID bath, & Mercury is too close to the sun. So...] It really only leaves us with one option if we want to become a multi-planetary civilization, and that is Mars. We could conceivably go to our moon, and I actually have nothing against going to the moon, but I think it is challenging to become multi-planetary on the moon because it is much smaller than a planet. It does not have any atmosphere. It is not as resource-rich as Mars. It has got a 28-day day, whereas the Mars day is 24.5 hours. In general, Mars is far better-suited ultimately to scale up to be a self-sustaining civilization."
As for toxic soil, http://www.space.com/37402-mars-life-soil-toxic-perchlorates-radiation.html
"The Martian surface may be even less hospitable to life than scientists had thought.
Ultraviolet (UV) radiation streaming from the sun "activates" chlorine compounds in the Red Planet's soil, turning them into potent microbe-killers, a new study suggests.
These compounds, known as perchlorates, seem to be widespread in the Martian dirt; several NASA missions have detected them at a variety of locations. Perchlorates have some characteristics that would appear to boost the Red Planet's habitability. They drastically lower the freezing point of water, for example, and they offer a potential energy source for microorganisms, scientists have said. [The Search for Life on Mars: A Photo Timeline]
But the new study, by Jennifer Wadsworth and Charles Cockell — both of the U.K. Centre for Astrobiology at the University of Edinburgh in Scotland — paints perchlorates in a different light. The researchers exposed the bacterium Bacillus subtilis, a common spacecraft contaminant, to perchlorates and UV radiation at levels similar to those found at and near the Martian surface. (Because Mars' atmosphere is just 1 percent as thick as that of Earth, UV fluxes are much higher on the Red Planet than on Earth.)
The bacterial cells lost viability within minutes"
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So the problem, likely real, appears to only be a major problem in sunlight, when strong UV light can cause the perchlorates to break down and further react. The plan is to build the base deep underground, for a variety of reasons (likely now including water ice near the equator). Escaping the UV light was a necessity, anyway. Knowing that the UV breaks down percholorates, forming chlorine and other things you won't want to breath, is probably another reason.
I think the perchlorate problem is relatively surmountable, just by going underground. I would make surface "terraforming" a problem. But that isn't on Musk's issues horizon. I think he just wants to get something going which can become independent and self-sufficient.
He's been clear going to Mars will quite likely kill a lot, if not all, the people who volunteer to go. But the American frontier held that risk, as well, in addition to providing a "clean break" for that time.
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