Don't buy from a pet store, you will be getting a poorly bred dog who may have tons of health problems, and you will be supporting the people who bred him that way. Find a good breeder or rescue one.
Rat terriers have lots of anxiety issues, please make sure to find a training class so he does not get fear aggressive.
Of course he will need to go out in the middle of the night. Buy a crate and a book on crate training, this will help. But all puppies have to go out every few hours at first, especially small breeds.
EDIT: This list is for Orlando. I don't know where in Flordia you are. But just change the town.
http://search.petfinder.com/search/search.cgi?breed=Rat+Terrier&tmpl=&preview=&name=&animal=Dog&age=baby&preview=&zip=Orlando%2C+FL