When you go to look at a puppy, you may not always be able to see the father as external studs are regularly used, however you should ALWAYS see the mother.
All of the chihuahua breed clubs reccomend that you never take a puppy under 10 weeks old but preferably 12 weeks. At 10 weeks your puppy should have had at least one if not both of their vaccinations.
At 12 weeks, the price should include, vaccinations, health check, KC registrations (if they have any some breeders decide to withhold the registrations which is different to not being able to provide one.) and insurance.
You should get a bag of the dogs food which they are on now too, a diet sheet, vaccination and worming records and don't forget to get a receipt for the puppy, you will need this for insurances, should anything happen to him or her (heaven forbid).
Make sure, as far as the breeder is aware, there are no serious hereditary faults in the lines.
Ask the breeder lots of questions, they won't or rather, SHOULDN'T mind.