Remember that if you foster and fall in love with your foster dog and it's a good match, you can adopt the dog! Fostering doesn't necessarily mean falling in love with a dog only to have it taken away and never seen again.
Other benefits to fostering are that vet bills and sometimes food and flea stuff is paid for by the rescue group while you are fostering. Also, if the foster dog situation doesn't work for any reason, a good rescue group will take the dog back and find a better suited foster home.
You get to experience dog ownership without having to take on full responsibility forever for that animal (unless you decide to adopt the foster dog.) There are even puppies to foster, although my experience is that fostering puppies is ALOT more work than fostering adult dogs. If you're set on a puppy, why not foster one? You'll gain experience and know better what type of dog will work for your family.
Also, know that you don't have to narrow it down to one breed. Mixed breed dogs can be great!