While there are some breeds that are reasonably less active than others, it really depends on the specific dog. Now that you have decided on a mature dog, it really opens up the options quite a bit. Going through a rescue group can give you the best advantage for making a good fit for your requirements. Most of them place homeless pets in foster homes and the foster family can give you the best description of the dogs needs (whether it is high energy or not, protective, sleeps in the bed, etc).
However, here are a few breeds I think you should look into: Standard Poodle (there is a lesser known medium sized ones called "Moyen" or "Klein"), Standard American Eskimo, English Cocker Spaniel, Clumber Spaniel, Whippet, Retired Greyhound, Soft Coated Wheaten Terrier, Kerry Blue Terrier, Cavalier King Charles Spaniel, SharPei, Tibetan Terrier. Ultimately, the right dog will find you. Best of luck.