I think that choosing a dog if a very difficult thing (and a great commitment) as I have been going through the same thing myself for 6 weeks to find the right companion for my greyhound.
Over the past 15 years I've had rescue Staffie X, Lab X, Greyhound and I now have a young Lab.
The staffie X was great in the end. He was about 3 years old when we picked him up, but very protective, needed a lot of socialization to sort out his issues and needed to be neutered and shown who was in charge for about six months. He became the most loyal friend to my previous partner that could have ever been known. He could tell when he was about to have a fit or if his blood sugar was low and come tell me! He was never trained to do that, he just choose to.
My lab X was almost perfect! Hardly needed training at all. She was great with everything except kids and whining in the car. I was pretty sure that she was dumped out of a car and treated badly by children. When I picked her up from the pound, all I knew about her was that she was spayed. No history, no age (she was between 3 and 5), nothing. First few nights she kept escaping out of my yard. I realised she had been trained to only pee and poo on grass! I miss her greatly.
My greyhound was just 2 when we got her. She was afraid of everything (including us!). She took 12 months of work to stop her peeing, pooing, chewing, panicking etc. She is now (5 years later) a well adjusted love bucket! She is still nervy of strangers standing over her, but as soon as they sit down she is all over them for fuss and kisses! She is great with cats and chickens, but needs to be watched if there are other animals running round! She "picks up" birds who are not in their own area (one of the ducks yesterday) but just holds them till we come over! She gets excited when the pigs or sheep run around, but she has no prey drive, just wants to play!
We adopted a chocolate lab last Friday! She has a great temperament and is settling well. Both our greyhound and two cats have no problems with her. Bad points so far are that she wants to play with everything.. ducks, sheep, chickens, phones, cat bowls, anything wooden, table cloths, blankets, plastic on chairs.. this list is endless! She chews anything hard and does have a habit of mouthing us when playing. I have now managed to dog-proof the kitchen for when we go out and we supply both the dogs with toys and hide chews. I am sure that with training and time she will become a great addition to our family.
What I am trying to illustrate is that every dog will need your time, energy and patience to become the dog you want. Find the right basics (temperament, energy level, etc) but be aware that you will need to nurture it, so that it will become the dog you want! Whether a puppy or an adult, these issues may well need to be dealt with and doing it in the right way will create exactly what you need.