Another option is a retired sheepdog. I never retire mine as I think it ages them, so they continue working in some shape or form til the day they pass. But I know other people who retire perfectly welltrained dogs which aren't able to manage the larger flocks.
Castlefarm is right - I train dogs and I am sure there would be plenty of people who help you train a young dog and provided you give it plenty of mental and physical stimulation there are no isses with collies. My partner and I started out rescuing collies and had no sheep and then ended up having to buy 200+ ewes to keep them entertained but we do have 7 dogs at the moment plus other dogs in for training.
Look carefully for responsible breeders with good pedigree ISDS dogs and insist on seeing the parents working before selecting a pup.
A good dog is a wonderful companion, work mate and friend!