I think you need to do a bit more research...
A 'collie type' from a farm - is either a back yard bred/puppy farmed dog that is not a working type at all, (but may be sold as such to unsuspecting pet owners who THINK they want that sort of dog) - bred from whatever dogs are around and may in fact be from a VERY small gene pool indeed, potentially, line bred from the owners own dogs over many generations.
Or a working type is likely to be a ISDS reg pedigree rather than KC reg, and again line bred from a pretty small gene pool of working dogs that are local to the breeder or owned by the breeder.
No reputable breeder of working sheepdogs is selling you a quality dog as a pet, they know you're not going to handle that unless you have working sheepdog experience and have land/sheep etc to work.
The idea that the 'good old mongrel' is a healthy dog is bunkum I'm afraid. Many are from tiny localised gene pools with people breeding just within dogs they own or neighbours/friends own, based on proximity, rather than health, conformation, whether that suits their bitch etc.
Quality crossbreeds do exist - but they are significantly harder to find than healthy, quality, well bred pedigrees.