Am I buying a sick puppy? Or just a naturally calm one?

I have been looking to adopt a new puppy recently, and found a yorkie-bichon frese mix that I absolutley adore. I met him yesterday, he was sleeping when I got there and walked right by him several times to play with other puppies. my fiance picked up the sleeping pup and we were with him for about 2 hours…he seemed shy and timid, but that was okay with us. He was snuggly and seemed to really like us…I was getting ready to sign the papers for him, and he threw up all over my fiance… It was disgusting to say the least. So I put him back in his pen and my fiance went to get cleaned up. as soon as I put the puppy down, he started pooping…now, I’ve had dogs before, and know what normal dog poop looks like (im blessed with this knowledge-lol-), it wasent 100% normal…just alittle runny/soft. This happens to my dog at home occasionally to when hes not feeling well. But he always gets over it in a few days. All the other puppies were jumping everywhere and wanting to play…and this one just wanted to snuggle and stay still. We put him on the floor and tried to play with him, but he just kept coming over to us to pick him up and hold him. I just wanted to make sure that he’s not sickly or anyhting…and when I say sickly, I mean something that wont go away on its own, or be very costly to have cured. …now, the store owner did say that a family came in earlier and was playing with him for an hour or two and that he might’ve just been sleepy from being with them… Which makes sense, and this man seemed like a very honest man and I dont think he was trying to scam us. He offered to hold the puppy for until monday (it was saturday when we went to visit), so he could watch him and see if its something serious or just a fluke. This place is an hour away from our house, so we can go down every day to check on him to see if its okay… We’re going down monday to see if he’s better, which in then case we’ll buy him, but if he’s not 100% better I don’t know what to do. like I said, my dog at home has been sick before but always got over it because he was healthy to begin with… I don’t know about this dog. My sisters friends yorkie pup was “calm, timid, and not very playful”, and they had to put it down because she was full of cancerous tumors - that scares me because I get sooo attached to my pets and I really dont wanna go through that. But I dont wanna leave behind a puppy im like in love with if he just has a little cold, you know? I really need some good knowledgeable advice. thank you sooo much for taking the time to read my question.