AGE - as I always take rescues my one rule is the dog must be between 3 - 6 years.
BREED - I would research the breed very carefully before choosing. Tbh, I have always had collie x's before I fell inlove with boxers..........I only got a boxer after walking someone elses for 2 years and talking to the owner at great length about them. I am lucky as I walk a few different breeds of dogs it gives me alot of insight into how they are....
TEMPERAMENT - because I walk and board other peoples dogs, the dog have to be ok around other dogs.