If you're only talking several months, or a year and a half which is a typical surface duration for a human expedition, then habitability is generally not a factor in site selection. The expedition brings their own resources for a mission of that duration, including water, which is partially recycled, and nuclear power. The exception is a reliance on Mars atmospheric CO2 from which to extract oxygen for breathing and return rocket propellants (using the C as well). Since the CO2 is everywhere, it has no bearing on site selection. You can see an example in NASA's Design Reference Architecture.
The main landing site selection criteria would be landing safety, which includes a limit on altitude, and, presuming that it is a scientific expedition, how well the landing site is expected to meet the science objectives. There will likely be an upper limit on latitude, based on the inclination of the return orbiter and capability of the rocket that will take them from the base to Mars orbit. A latitude limit is also useful for bounding the requirements on habitat thermal control. (Habitats include roving vehicles and spacesuits.) The science objectives quoted are usually related to the search for evidence of life that had originated on Mars.
Other constraints may derive from planetary protection concerns, in particular protecting Earth from putative Martian organisms that the crew might bring back outside of sealed sample containers. From the report:
In order for humans to explore Mars and return to Earth safely, it
will be necessary to identify sites on Mars that are free of hazards
to the Earth’s biosphere. This is because astronauts on the martian
surface inevitably would be exposed to local martian materials such as
dust, and the plan is to return the astronauts to Earth at the end of
the mission. The astronauts are therefore a potential vector for the
transport of martian dust, which must be shown in advance to be
sufficiently safe. The Space Studies Board has recommended the
designation of Zones of Minimum Biological Risk (ZBRs) that are
regions demonstrated to be safe for humans. That is, astronauts would
only be allowed in areas that are demonstrated to be safe. For the
initial landing site, such testing would probably have been performed
as a part of the precursor mission activities, which may include
analysis on Earth of returned martian samples, particularly wind-blown
dust.
The same planetary protection concerns require the protection of Mars habitats from human-borne microorganisms:
The strategy that was adopted for the current DRA envisions targeting
the human landing site that would be located within an area that is
already known to be safe to humans (a zone of minimum biological risk)
and in which microbial contamination would be permitted.
If humans have to avoid the very thing we might be looking for, it would put into question why we would send them. If such constraints cannot be avoided, a human expedition might be limited to the search for evidence of life in the fossil record.
