The best way that I know of to try to contain and control the human factors in a project is to have a clear idea of your own processes.
Start with some Domain Driven Design, work with the users and help them to understand their domain and the business processes that surround the domain. Often developers are far better at abstraction than the managers/business people so we can often help them to understand their own domain.
Build up a set of acceptance criteria, these form your tests which actually form your spec.
Once you have an idea of the above you know much more about feasibility and how long it will take (and even if the technology that has been specified is the right one)
As for approaching new technologies, start small, build a proof of concept and make your mistakes there rather than on production code. There is a huge amount of best practice on the web and places like StackOverflow are good places to start.
I would suggest working in an agile fashion, get the project owners to prioritise the work that needs to be done, work out what is needed for the next two week sprint and deliver it (which may mean stubbing out a lot of functionality). They ll tell you when it is wrong and it may influence their own decision making.
Don t view the entire project as a nasty whole, break it down into deliverable sections and one step at a time.