If you're 'just looking' and 'researching' why have you denied every turn that doesn't fit into BACK OFF?
'Oh, vet may not be accurate..'
'She sounded really nice..'
'Dogs are in a pen, not just outside..'
If you are honestly just researching, go for the bad side as well as the good side.
Also, why on earth would you want to purposely buy another dog with knee problems? Surely you don't want to see your dog suffer?
Not meant accusingly, that's a genuine question. To me it appears you are almost saying I don't mind if the puppy has health problems.