Hmm having just adopted our 2nd I have mixed views on this.
My older (only 2) loves other dogs. We have Fostered (not while or 2 year old has been here) in the past so know how to make very slow introductions. She did not take to our recent adoptee for quite a while. Despite loving other dogs she wasn't impressed and did go through periods of stress.
It took 6 weeks and now they get on, but not all the time. The older one needs time away from the puppy, so puppy comes and sleeps in her pen while I work and the older one stays with dp.
We are getting there but it's taking a lot of time. Not sure I would want to go through this with an aging 14 year old. Especially, one that is human focused.
Our oldest is bonded to us and there is jealousy, but she is happy with cuddles off anyone. Not bonded to one person only, which is a breed trait, for the dogs I have. But we have always worked in that. We all feed her, all walk her, all play and spend time cuddling her.
A dog new in the house will need alot of your time and attention and that may be a cause of real issues for your older dog. And your older dog liking other dogs of the same breed doesn't mean they will be happy with another dog permanently there.
My other issue is with people seeling the dog. There's a reason for it. Change of circumstances is often the reason being used. But is often a lie. As are the 'oh he is great with dogs/kids/other animals' or 'absolutely dream dog, such a shame'
Most people, who are good owners, genuinely trying to find the right home for their much loved well behaved pet, would work through a rescue to ensure the dogs new home and family is properly vetted. Not selling it to someone they have met once.
That all said, I love having 2 dogs. Its been worth the effort of getting my 2 together. But as I said, I wouldn't have done it had my older one, been of an age where they were slowing down into old age. I think the stress and upset could be too much for them.