What dogs shed the least and are good, calm house dogs? Which breeds live the longest?

I am interested in getting a dog. I live in the country so there is little traffic. I want a dog that doesn't shed too much, will alert me to anyone coming to visit,is smart but not hyper. I want it to go for walks with me but not be so big that it is pulling me along, rather than walking beside me. Medium sized, I think. It could be a mixed breed from an animal shelter

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