The Facebook & Twitter 'Simple' Connect plugins I linked to are called 'Simple' - but it seems they have many features, including sharing on facebook and tweeting options, it looks like those two plugins will be all you need for facebook and twitter social network integration.
This is a very hard thing to come by - how much to charge someone - personally, I like to think about what my time is worth, how complicated the work is, and how long it will take. I will then think about an hourly rate (for design/page slicing/html/css) + extra complicated work (which for you is WP theme development and how much you think learning this is worth to you) * how many hours I think it will take.
(Hourly Rate + Extra Work)*hours = fixed cost. Giving them a fixed cost at the beginning can help them determine if they can afford it, or if they can afford more. Of course, this being your first job, I would suggest having quite a low hourly rate as you haven't got the experience behind you. Finding the right rate for you can take some time. If they ask for extra on the site after you have completed things, just adjust the cost.
There's most definitely themes out there, but if you want to get into freelance work, you best build up your portfolio and take every opportunity you can to do that. This is one of those times. Plus, buying a theme is the easy way out.
There really is no end to what wordpress plugins are available, here's one example of a email subscriber plugin for you.
Best of luck to your first job and welcome to the world of web design freelancing!