First of all - I would like to commend you for really thinking through your choice of dog. So many people don't really think it through and end up with a dog that is totally wrong for them. Many times the dog ends up either neglected or turned into a shelter. I get so frustrated when I see an older couple leaving with a high energy puppy when there is a wonderful 6 year old mutt in the shelter that would be so content to follow them around and lay at their feet. Or the people with the three young kids taking a dog that flinches at the slightest sharp movement.

That being said, since you seem to be more interested in a type of dog rather than a specific breed I would look into mixed breed dogs from a shelter or rescue organization. There is a great book by Brian Kilcommons called "Mutts America's Dogs", which discusses the personality trait of mixed breed dogs.

If you feel that a purebred or mostly purebred dog is better for you, his book "Paws to Consider" is also a great reference source of personality traits and can help you choose the best dog for your lifestyle. If you go that route - a reputable rescue organization in definitely the way to go. Even among the same breeds, there are different personalities and a rescue organization can help you choose the right dog.

If you truly decide you want a dog that you can work with in agility or such, herding dog mixes might be the way to go. If they are mixed with a breed that is a little calmer in nature they might fit in well. You would have to be careful with herding breeds, however since they have a tendency to be prey driven and could be a problem with a cat in the house. I have a cattle dog mix that I have to watch constantly when my cat is around. Again a rescue organization would be able to help choose the right dog for you.

I am also partial to German Shepards or Shepard mixes. They are very intelligent and do very well in obedience. I have also found that they are high energy when you want to be (i.e. in the yard or out for a run) but couch potatoes in the house. Since they are easy to train, they can be taught to bark once and then be quiet on command. Shepard mixes would easily fit in your weight requirement. Again a rescue organization would be able to help you choose the perfect dog for your lifestyle.

Good Luck