the first thought that came to my mind from your post was a greyhound but you said they dont appeal to you. exercise doesnt necessarily mean walks. my border collie mix and my rottie mix got about 40 minutes a day but we would play fetch, do some training (shadow loved agility so we did that with him) and play chase, find the toy and other things. sometimes mental activity wears a dog out as much as physical.
by the way, shelties are known for barking from what ive heard, i dont know if that will be a problem.