Most good breeders offer the one year guarantee. Also, some require you to take your pup to your vet within a certain time frame. Also you might check references on the breeder to see if past customers were pleased or not. It sounds as if the 4 month old might have been small so she kept her. Did you ask why she hadn't sold her already?
I have three girls, all from different people. Rosie has IBS. Mikki has LP and so far Lily is great. Rosie is 6 lb, Mikki is 4 lb, and Lily is projected to be 4-5 lb.
You also might look into getting insurance for health problems that might arise. As far as being weary of getting her, I tend to follow my heart. Then deal with the problems if they arise. I took both Rosie and Mikki without knowing anything, no health guarantee, etc. Lily was purchased from a breeder who I researched heavily. I love my girls and wouldn't trade them for anything. So my advice is follow your heart.
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