>> i want it to be a puppy when i get her, but how will i know if she is suitable for breeding at this age? <<
You wont know when she is a puppy, study the breed you are interested in and when you think you can tell a good one from an all-so-ran, buy a puppy for a breeder who breeds the best you can find. Then rear her with the best of everything and hope when she is old enough she will not only pass the health tests well but will fulfil her potential and do well in the show ring or the field.
>> will the breeder let me breed from a dog they have bred? <<
If you make it clear to the breeder that you wish to breed they will sign to say they will remove the restriction if the bitch is health tested and is worthy of being bred.
>> how will i stop people from buying them to breed from if i do not want them to be bred from <<
You will do the same thing, tell the purchaser that you will remove the endorsement if the bitch/dog is worthy of breeding at 2 years, put it in writing and both sign it and both have a copy. Or tell the purchaser that the pup is not to be breed from and have them sign to say they understand that the restriction will not be lifted.