I have to say, I believe you're thinking about this completely the wrong way round. Don't start with the plugins you need to do the job: approach it from why people would want -- no, *need* -- to be using your web application.
People will only use a site if it fulfils their own need. Work out what services you MUST provide, and provide them. Then build from there, but take each enhancement on its own merits, and always keep the goal, rather than the technical means, as your driving thought process.
Nobody on here will be able to say whether one login/authentication system generator (for example) is better than another for your project -- only you will be able to decide what's best for you. And the only way you can do that is working out what your application should be doing. Working out how to do it comes later.
Scott
