for 12 years, and she was a delight. All terriers are strongly territorial, and some are prone to jealousy - we wouldn't have left ours alone with a very young child. If you buy this puppy you need to understand that it will bark whenever the doorbell rings, and it's extremely difficult to break the habit - a friend of ours is a specialist dog trainer, and he couldn't do it.
Otherwise, you should know that pedigree Westies are prone to eye problems - they can get a condition known as 'dry eye', which means their tear ducts don't produce enough moisture, and it leads to infections. Ours suffered from it, and it involved a very expensive (successful) operation.
All pedigree dogs can exhibit weaknesses; if you want a dog that's free of predispositions to illnesses and conditions go for hybrid vigour - get a cross bred dog.
I wouldn't worry too much about Kennel Club registration unless you plan to breed from your dog - it's more important to see the puppy with its mother if possible. If there's a litter, look for a puppy that comes to you with its tail wagging, and avoid one that cringes away from you. A bold dog will be far less trouble as it matures.