I don't believe that certain breeds carry certain temperaments. I have two chocolate Labs. They could not be any less similar. The only similarity between the two is that they're both brown, and that's where it ends. In terms of personalities, they're on complete opposite ends of the spectrum.
As for buying or adopting a mixed breed, it's a good thing. Pure bred dogs have been interbred so much to achieve particular AKC conformation characteristics that they end up with a lot of health problems. E.g., vision issues, joint issues, breathing issues, etc. Mixing breeds typically breeds out those health issues and you end up with a much healthier dog. I'm all for mixed breeds.
Cruisin'girl, my boy Lab, Blair, was $1,000 and he didn't come with papers. My grrl, Stella, was adopted as a rescue dog for $400 and she also didn't come with papers.
Anyway, please don't shop, adopt. The vast majority of dogs are in shelters and in rescue orgs not because they're bod dogs, but because so many people buy them and then decide they don't want them. E.g., my Stella was put up for adoption because her previous family neglected her for the past 3 years once they started having kids. Unfortunately, that's the usual reason people dump dogs.
And if there is a particular breed you're interested in, there is a rescue organization.