Alan Bradley does 'sheepdog experience' days in the Lake District and also has dogs to sell.
http://www.lakedistrictworkingsheepdogs.co.uk/sheep-dog-handling-courses/I can thoroughly recommend the experience day to give you a feel for handling a dog, and if Alan has one he thinks may suit you you can try it out there.
Alan uses his own dogs and those he has for sale on his courses, so they get used to working for different people. He trains his dogs in different voices and accents so that they find it easier to adapt to different handlers!
I have always had dogs but had never had a working collie working sheep before (done agility and obedience but not herding sheep.) I got given a dog that 'knew what to do but didn't really have any commands' by a local friendly farmer, and, knowing I needed one I could command and that our farm needed at least two working collies, I also bought a part-trained one from Alan. Both dogs were a godsend, in different ways!

When I was looking, dogs that could do the basics and weren't puppies any longer were £500 - £1000, fully trained dogs with farm experience were £1000 - £2000.
I do now know of people who can supply part-trained dogs at about £300; they are probably not registered but will be of locally-known working stock. They mostly will be live-out dogs; a lot of working collies don't settle well as house dogs, although most enjoy coming in for a bit of a treat for 20 minutes now and again.
Others will recommend Barbara Sykes' book, "Understanding your Border Collie"; I do now have a copy and am reading it. I like what I've read so far and I think for someone wanting a collie to be part companion / pet and part worker, it is probably a very good book to read.
You are right that some collies don't cope with small children very well - for me, that'd be a reason to get an older one whose temperament is already formed. If you got a pup and it decided later on it didn't like children...
The other advice I'd give is find a local sheepdog-handler-trainer who will work with you and your dog. My side of the country, I'm lucky enough to have Derek Scrimgeour, we've Katy Cropper in the county too and Tom Longton just over the border in Lancashire ; I'm sure I've seen someone recommend a one out your way but I can't recall who it was.
Good on you for doing some research and thinking it through, and getting some advice, before taking on one of these amazing, but rarely easy, creatures.
