I don't think anyone will have the cash to design, build and launch so many modules needed for assembly in space (way too heavy for just a few launches, let alone sent in one piece).
The radiation protection required to let 4 people survive the trip to Mars alone clocks in easily at dozens of tons total weight, even if their living space is kept to an absolute minimum (psychological limits for trip duration).
Additional radiation protection against solar flares will require a specially hardened shelter in the spacecraft, taking several tons of additional weight all by itself (short of anyone developing a star trek-like shield in the meantime).
Then add oxygen supplies, food, water, regenerative supplies, sensors, minimum required equipment and spare parts for emergency repairs etc.... And that super-heavy construction still needs all the fuel to be carried along.
That's like sending something far heavier than the ISS to the Mars (which would be entirely insufficient, especially concerning radiation protection). And we all know how much cost, effort and time went into that alone.
IMHO sending people to Mars with current technology is severely far more unrealistic than sending a man to the moon. Both could in theory be achieved with todays technology - but the costs to mars would literally be near-insane. Easily far more than all international space agency budgets combined.
I just don't see any nation or even group of nations being even remotely able to channel hundreds of billion dollars into such a project, definitely not under current (and forseeable) economic conditions.
Considering i.e. NASA's budget of the last years, they could expect a launch not before the year 2050+ unless we see major technological breakthroughs (or the USA somehow growing filthy rich) in the meantime...
(additionally, I haven't even considered the numerous robotic mars missions needed to construct a 4-person habitat to afford survivability on mars for a few decades as a launch prerequisite; that as well would suck in ALOT of time and money).
Would be a pretty bad idea to send 4 people to Mars just to see them dying within months.
Therefor, to me these plans seem more like fairytales coming from dreamland (I very well remember being more than surprised hearing George W. Bush making that original announcement, I was like... WTF?!).
Would be cool if someone could pull it off but I don't expect to live to see that day.
PS.
IMHO, a far more realistic (and yet still unaffordable) plan would be to build a real, expandable and permanent space station with assembly or even construction capabilities as an autonomous launch platform.
As far as I can see, that would be man's first, actual and real step into manned space exploration with reasonable and lasting potential.
Maybe one of the keys of future space developments would be completely new generations of materials - lighter and significantly more resistant than anything we know and use today for our superstructure/hull designs. A material that preferrably would withstand the hyper-kinetic impact effects of typical small space debris or objects. To date, mankind has no such material, making a single such collision potentially catastrophic in nature.
What you propose is fairly realistic. However for specifics on the Mars issue review Zubrin, Mars Direct.
I am afraid we will...perhaps have been...encountering a fundamental problem in our planning for manned exploration of space, with some exceptions.
That is we take a planning horizon for manned exploration given our estimates of future robotic capabilities. A third of the way through that planning horizon, the entire plan has to be scrapped or revamped as then-current robotic capabilities were evident. Over and over.
But yeah, a long term presence in orbit...think an economically self sufficient presence, which would mean a commercial, not a government system...would be the first true step in going off planet.
As far as materials, all you really need to protect from impacts is a water or ice barrier. It is particularly well suited since in vacuum, ice sublimates to gas, thus leaving no cloud of micrometeorites of its own making. Ice would only have to be contained within plastic bags that could withstand the vapor pressure of sublimation, and shielded from solar, of course.
And in that above paragraph is a hint of the essential problem in long term LEO or anywhere else human presence: Materials. Where do you get a thousand tons of water? Launch ten or twenty at a time. That's one answer.
Moreover, any such system could not be at LEO, but would have to be higher, likely way higher. Ultimately, one of the Lagrange points would be perfect. As the satellite weight increases, manuevering the station becomes virtually impossible. Hence it must be in a permanent orbit, which LEO is definitely not, being good for a decade or two.
The general vision for this was laid out in O'Neill, "High Frontier".
