A main problem is providing astronauts enough food and water, and other necessary supplies for a long journey to Mars.
Considering that it took around 10 years to fully construct the ISS for habitation by six. Spend 10 years assembling your Mars spacecraft. Also assemble an accompanying rocket with supplies. In the meantime, launch a couple of test missions, with the chief objective being to see how fast we can get a test vehicle to Mars and back. Put some plants on them, some detectors and such. It will be money well spent, since this will help determine how to get humans there Fast. Less bone density loss, less exposure to radiation and etc.
Let's not plan for a 6-month trip. Let's plan for a six week trip, or less. Let's be optimistic and get this done. Werner Von Braun would already have it done by now. Develop your fast-boost spacecraft. Not a tortoise, but a hare is needed to get us there and back.
Let's plan for a shorter time duration trip to Mars, to save on supplies and astronauts radiation exposure. Boost them three times as fast towards a Mars approach orbit. They would get there, have a month to explore, and could lift off in time to make a fast Earth approach return. We could test this with a pathfinder mission specifically designed to test a fast-speed trip to Mars, orbit once and return to earth. This would be well worth the cost, to prove the idea that you can get people there in a shorter time span.
This Pathfinder mission could test the concept of a rapid trip to mars and back. You would want to take along some plant life or even animal life to test the effects of radiation during a fast trip. The caveat, of course, is that we would need to boost up a lot of fuel for the trip there and back, to boost the spacecraft to, say, 250,000 MPH. This does not seem impossible to me, since we have already sent craft speeding to Pluto at 50,000 MPH.
For a Mars mission, to boost up all of the extra fuel needed, along with the requisite cryogenic storage systems would certainly be expensive. But it seems to me that would be less expensive than keeping a 6-person crew alive for at least 18 months, if not longer. It took us over 10 years to assemble the ISS in LEO. If we spent around that time assembling a huge fuel storage cache and a Mars expedition craft, we could achieve a fast trip there and back, and bring back healthier astronauts as well. How about it – let's get there sooner. We can do it if we try.