I've given this some more thought, and my advice differs depending on whether you have had dogs before, of what type, and what you did with them.
If you aren't used to having your own dog, then personally I'd steer clear of the one you've been offered and, if you like the idea of a pup in the family, look out for perhaps a collie x lab or something similar (accidents occur, if you put the word out, some farm somewhere will have a collie bitch in pup to a lab sooner or later; or someone who has labs and lives near a farm will have a lab bitch in pup to the local farm collie.) If there's a bit of collie in the mix, it will probably have a bit of sheep sense (though not guaranteed, not even all pure collies are interested in, or skilled with, sheep), and hopefully not be so obsessed with sheep that it can't behave unless it's working every day.
You'd still need to get training for you and for it, of course.
If you have had dogs before, and are used to training them and working with them even if it's only obedience, flyball, agility or what not, then actually I would probably still steer clear of the pup you've been offered.
Working with working collies working sheep is not at all like any other handler-dog relationship I had experienced. I think I would be inclined to see if there's a local farm collie not quite up to the job on the farm, but would make a good smallholder's dog, and help out a bit but not be obsessive. You'll learn a lot from working with an experienced collie, and it'll already have learned about hens and so on. Once you've had such a dog, you'd be better placed to train your own pup next time. You would of course take great care in adopting such a dog - there could be a number of farmers with less-than-useful dogs happy to palm them off on an unsuspecting smallholder! But there will also be genuine opportunities - you would just need to be quite robust in rejecting any dogs which didn't seem suitable. Most farmers would ask that you return the dog to them if it doesn't work out - if they don't do this, take care!, and ask for a one month trial initially.
On the type of the pup you've been offered, the Huntaway is bred to move large numbers (up to thousands) of sheep by barking at them, and to work with them in close quarters (in the pens) by running along their backs. It's an interesting component in your dog if you have hundreds or even a thousand of sheep on a fell, but I really think it's probably not the ideal makeup for a dog to work a small flock of primitive type sheep.
You don't say what rare breed your sheep are; if it's a dopey great longwool type, the Huntaway would probably be quite good, as the sheep don't always see too well! However, if they are a primitive type, then IME they need handling quite sensitively by the sheepdog or they'll just scatter, scale fences and so on. (My own collie dogs have learned to hang well back from my primitives, and to be very calm, quiet and patient.)
Sorry for the essay!
I'm not sure whereabouts in Devon you are, but if you can get to David Kennard's displays at Borough Farm, you could have a chat with him. Not only might he offer you and any dog you do get some training, but he may know of a suitable dog, either now or in the future. He might also be able to help you assess any dog you are offered. (I say 'might' because sometimes he's busy with filming or other projects, but if not then he's usually happy to help. And if he is too busy himself, he'd be able to suggest someone else you could approach.)