This guide to buying a puppy is well worth a read, as is the same site's list of questions to ask a breeder.
Health testing wise at an absolute minimum both parents should have been hip scored (with results under their respective breed averages; 12 for the BC, 13 for the GR), have current BVA eye tests (these are repeated annually) and gonioscopies (repeated every three years), the GR should have had their elbows scored (with a result of 0), both should have been DNA tested for (or be clear by parentage of) Neuronal Ceroid Lipofuscinosis and the BC should have been tested for the MDR1 gene defect.
Ideally the GR will also have been DNA tested for (or be clear by parentage of) Brittle Bone Disease, Degenerative Myelopathy, Ichthyosis, Muscular Dystrophy plus two types of PRA: GR-PRA1 and GR-PRA2, and the BC parent will have been DNA tested for Cobalamin Malabsorption, Collie Eye Anomaly, Raine Syndrome and Trapped Neutrophil Syndrome.
Whilst puppies of this cross can't be affected by some of the conditions listed above (because they're recessive and only present in one of the two breeds) a good breeder will still do all the tests because they should want to know whether they're potentially producing puppies which are carriers.
Don't just take the breeder's word on the health testing, ask to see all the official documentation. Any half decent breeder will be happy to show you this and talk you through all the results. If you have the dam's registered name then you can check for health test results here on the KC site however the KC don't currently list all available tests so seeing all the official documentation is preferable.
Does the sire actually work? There's a pretty big difference between a pet bred working sheepdog type and a genuine working sheepdog. And has the breeder done anything with dam? Showing/sports/working/training qualifications/etc.? Resource guarding can be a big issue in some lines of GRs, I'd want to be happy the bitch has somehow proven her good temperament.
The FB group Wolfiefan mentioned is brilliant, I highly recommend having a read through their Congratulations on Your New Puppy set of files.