What to look for?

Hi! I have been reading a lot about goldendoodles and found some in my area for sale. I'm going to meet them on Saturday (and possibly bring one home!).

However, I am a first-time dog owner... I've never picked out a dog before, so I thought I'd ask for some tips?

I read on the site to flip them on their backs and try to pet their bellies to see if they are submissive to you or try to flip over right away. So, I'll do that... Any other tips you can give me? Or something I should look for in these puppies?

I was also curious about names. The dogs I'm looking at both already have names, none that I particularly like... Is it ok to change the dog's name? They're about 8 weeks old now.