What is a must have lens for the Nex-5N? Any brands other than Sony

I was hoping someone could help.....

I don't have much money left to spend after buying the NEX-5N this year, it will have the kit lens but I would love to get a really nice lens for portraits of my family and my doggies, especially my new puppy. I can afford something around the $300-$400 mark.

I was wondering if I'd be able to get some cheap lenses off eBay and use some sort of adapter to get the lens to fit the NEX5N. I will be doing video with the NEX5N also.
Thanks! I'm so excited, my Nex-5N should be here Monday!