This is an enormous thread, and I'm new, so apologies if this point has been raised already:

One thing many people say is that "the puppy chose me" - i.e. The puppy was boisterous and friendly, and was jumping all over them 'asking' them to buy him (/her) when they first went to meet him.

This may seem cute to us, but this is not the way to choose a puppy. Such behaviour is displayed by a dominant pup, and could very well spell trouble in later life - especially in the 'guarding' breeds of dog.

You should really choose a puppy who is calm, well socialised in the pack with his siblings (and with his human companions), and who is happy to be stroked and petted, and doesn't spend all his/her time shying away in a corner - which could also spell trouble in later life with a fearful dog.