This dog resembles my uncle's Walker hounds a lot. -love those dogs! (haven't any idea what breed/mix of breeds your dog is, but the vet may can help with that a bit) -anyway, it's adorable. I hope that everything works out well!
There is no telling what type of abuse or neglect that the dog may have been subjected to previously (prior to being adopted into your loving home environment). That alone may have some bearing upon the behavior. You are wise to be very cautious. Purchasing from a shelter or rescue is wonderful...