I think my cat would like a Miniture Dachsund to play with

I would only get a dog if you have a chance to let them meet a couple times. Dogs and cats have very different needs. While Hank sounds very sociable, it could because other cats will leave eventually while a pet dog won't. Don't forget that dogs and cats socialized differently. While there may have been many cats, they are individual predators that tolerate each other because there is enough food to go around. Dogs are pack animals in comparison, we've even breed that trait strong so that they've become man's best friend. A dog might want to assert itself as alpha and that might curb Hanks natural social personality. Plus a dog might scare off the other cats and isolate Hank further. If you go through with it, you need to make sure that they get along and show no aggressiveness towards each other and that if the dog does become Hanks only friend, that they enjoy each others' company. Which can be hard to gauge since cats can be rather standoffish with anybody at first.