I'm not biaised, but if you go onto any of the Mountain Rescue Dog Team websites and look at their dog team photos, you will see that most are Collies, also some Labradors (now I am biaised!)
You will also find a lot of collies in rescue centres, unfortunately - so that would work well for your story.
As the others said, keep us informed about your book, we'll all be interested in reading it. I hope your research will include volunteering as a dogsbody for an MR team's training session - that's the fun bit!!
Judith