Hi L.,
I think that your concerns and fear are natural and normal. As a previous nanny for many years and now a Mom to an 11 month old I can give you some of the many benefits of having a nanny. A nanny can devote quality and one on one time with your child. If you allow it, a nanny can take your child on "field trips" such as parks, museums, the zoo, etc. A nanny is solely responsible for your child not a classroom full of children. Nannies are often warm, creative, energetic, and most of all love children.
I would recommend requesting background clearances, and requesting references of former employers. I would also spend time with the nanny and your child to see how the nanny interacts with your child and how your child is responding to the nanny. As time goes on your fears will be eased if you see that your child is excited when the nanny comes and if your child is sending positive signals about the nanny.
I realize this is a touch decision but there are many positives to having a "good nanny."