It's appealing to see ourself "saving" an adult dog from an animal shelter but dealing with a dog with a troubled or insecure past can be very challenging.
The good news is that you'll be getting a full size dog so you know what to expect in that regard. He's likely to have at least some rudimentary training, perhaps from shelter personnel.
The worst thing that can happen is that dog and owner are not well matched. Inexperienced shelter personnel might place a strong-willed, dog with an elderly couple or a busy couple with young children.
Sometimes, owners over-estimate their own abilities and under-estimate time commitment to handle dogs with tricky or troubled personalities.
Many older dogs entering a new home will not behave well. Expect problems like whining, barking, destroying things, and toilet mistakes.
That's because dogs are anxious and do not feel safe in new home. Do not take this behavior personally and try not to become angry.
Please recognize that concept of gratitude is a human one. It does not exist in dogdom.