Both our dogs were pound dogs, rescued by a shelter on 'euthanisa day' and then adopted out. They had heartworm tests done by the shelter and were given the all clear.
When we adopted Molly, we had her home for a few days and then she developed kennel cough, the shelter (RSPCA) arranged for us to take her to an emergency vet on a Sunday and they covered all the vet and medicine costs. I was a bit worried about Meika also catching KC but all her vaccinations were up-to-date and thankfully they were working Molly had been vaccinated but wasn't out of the 'safe time zone' yet, but in all honestly, there are much worse things to worry about than KC - its very rarely a bad illness and they usually recover within a few days
Our local pound does not adopt out to the public, they rely on rescue organisations to save the animals, otherwise they are ethanised. They do, however, take 'expressions of interest' from people and pass their details onto the rescue groups, so the rescue (usually the RSPCA but some smaller rescues too) will come and collect the animal, contact the interested person, carry out all vet work (including heart worm testing) and then adopt the animal out to the interested person.