I think it sounds like you’re not ready for a dog yet. If you can’t trust adult dogs because you don’t know them, you need to work on that first. When you walk your puppy, it will come into contact with adult dogs that you don’t know, a certain level of trust is required to let the puppy socialise.
Volunteer with some adult dogs and I’m sure you’ll soon see how amazing most animals are once they have some love. Then you will be more ready to get a puppy.
If I was selling you a puppy and you appeared afraid of the adult dogs in my house I would not be reassured about your ability to meet the pups needs re socialising with adult dogs therefore would probably not sell to you just yet