I am looking into getting a dog soon. What are some opinions on dogs by knowledgeble patriots?

The requirements are few:

Dog has to live indoors.
Must be compatible with children.
Must learn to accept another dog (my daughter has a little ankle-biter)
Hopefully, can alert me to possible intruders.

What are some smart, child friendly breeds out there that will still make a good watchdog? Any thoughts?

Oh, and money is a problem, haha.