Yet more ideas:
Most design ideas for starships seem to incorporate huge ships just to support a few hundred people. This to me is silly. I think less is more.
In my mind the best way would be to make a starship as small as possible, and have as much as possibly operated autonomously (by robots etc).
Theres alot of worry about long duration missions is how a handfull of people would stand being couped up in a small space for years on end.
But then again for most of human history mankind has lived in small nomadic groups or more stationary family clans. In many cases never venturing far from where they were born at all.
Until the begining of this century small groups of hardy explorers endured months isolated on sailing ships to travel around the world. Sometimes not setting foot on land for years.
Even the average individual in a urban city has a very small network of friends they regularily interact with anyway. And nobody I know who shares an apartment with others has any long term problems living in the same small space.
So it seems we could get away with a skeleton crew in a small starship. Smaller means faster, which cuts journey times to another star from 20 years plus to 10 years or less.
To give the crew plenty to do (there would be alot of science to do but they need to play as well) a powerfull virtual reality system could give them more room to "play in". Acting as an escape from the confines of a small habitat.
There is a risk with a long duration starship journey, is that the next generation of faster starships will overtake you, or that technology back on earth will race ahead and the crew miss out on new things. To beat the first problem you'd have to have an agreement that if a faster ship can catch up it rendevous with it and collects the crew.
Data such as news, movies, digitsed books, TV, new software and games could be beamed out to the starship so they could keep up to date (however they would eventually be many years behind because of the imense distance). Perhaps plans and specifications for new devices (such as microchips and other machinery) could be transmitted too, and with an assortment on manurfacturing equipment and raw materials the crew could update and improve their starship.
A colony vessel need not carry all the living people needed for a breeding gene pool at the destination. As long exposure to interstellar radiation - even well sheilded would have damaged the DNA of the crew. So they could perhaps carry millions (perhaps billions) of samples of DNA encoded digitally and a DNA replicating machine. The recreated DNA - saved from damage by radiation, could then be inserted into blank eggs to create whatever species of creature you wish
When the vessel arrives at the destination star system, manurfacturing robots and equipment could mine resources and build new habitat and build more starships to go on to more systems. The habitats, settlements and vessels are populated by the newly born people who carry out the task of building a infrastructure in the new star system.
This is the shotgun approach to colonisation. With all the extrasolar science, interstellar exploration, and colonisation all achieved in one shot, rather than sending lots of unmanned probes and follow up manned missions. Just about every field of science would benefit from a effective mobile laboratory sent out across light years of space.
At the current exponential rate of progress (computers double in power every 18 months, robotics has come further in 20 years than mother nature did in 3 billion years of evolution). In about 20 years time we'll have the technology to build self-replicating machines that could assist on such a voyage. Nuclear fusion, solar sails and other such star drives have yet to be developed and are the major hurdle.
If we sent these machines to our own solar system, the could manurfacture anything, using the power of the sun or nuclear power, and send it back to earth. High power high quality electronics and materials could be raining down in capsules.. for free.
Its such a worthy investment in the future of our race and planett its a project all of mankind should work on. 200-300 billion dollars worth of investment would return almost infinite rewards.