I would go for a puppy as my sole companion and partner. The bond is much stronger, we get to live together for more time, and it is an awesome chapter in lives of dogs in general. You can much easily find a way to live together, older dogs can have much bigger problems getting used to new enviroment. And of course, it is the prime age to teach them some behavior so you both are safe. We have had many dogs in my family, and the dog that I was taking care of straight from breeders, while rest of family happened to have a holiday planned, still has deeper bond to me, than to others, even three years later, even if now I only visit them once a month or so.
However, really can see myself opening something like a private shelter for any dog that has been left behind and wants to join life of some human again. In my country, there is quite good service of animal shelters, but more east, in countries like Romania, or the whole Balkans, there are so many stray dogs that nobody cares about, and many of them need help, or at least some love. It was breaking my heart going around them and only being able to at least leave them some water and food to make at least a few days better. So, that I can see as my plan for "retirement", or really any time of my life when I would be self sufficient enough to be able to take care of more beings at once.