Angora, I really admire the big heart you have for thinking about bringing in another animal out of the goodness of your own heart. Did the owner say what she meant by "get rid" of the animal? Maybe she intends to just give him away to a friend or family member. Sarita made some very good points and it very well could have been a way to get you to buy him. That aside however, ideally I too really like for people to adopt their animals and "bunny date" at a shelter for pairing.
What are the reasons you want the rex? Is it because you really love him? (I think the resounding answer is yes.) Is it because of the guilt trip the gal at the store is giving you? You might want to write down the reasons and compare them. As for "rescuing" him, that could be a yes or a no. He could be headed to a shelter to begin with but on the other hand, yes you are helping her "business" of selling rabbits and pet overpopulation.
I too would love to take in every bun that I see, and would like to rescue them all. However there are some other things to think about: Would you be prepared to keep him for life in a separate cage and area from Henri if it does not work out? Are you financially able to take care of him, especially if he gets or is sick? Are you able to afford to have him neutered right now? Do you have the room for another cage? Is there anyone else in the house that needs to be part of the decision? From what I recall you have quite a few cats. How will they react to another animal in the house? The addition of this guy would mean another litter box and a cage to clean and it may stay that way if he doesn’t bond with Henri. Are you able to handle that possibility? Bonding can be very long and very frustrating, are you mentally prepared and have the time for it? Do you have a neutral place try bonding? Do you have a set up for it? Do you have the means to do so?
To answer your question no, there is no guarantee that Henri will bond with anyone but the chances are much higher if you take him to a shelter. There would be a variety of personalities to try him with, not to mention the bunnies there would already be fixed, litter trained and ready to go. Whereas you’ll need to wait to try him with Henri.
I know I haven’t given any yes or no answers because this is a decision that you need to make on your own and take some time to really think about all the "what if" scenarios and if you are willing and able to accept any possibility that comes up and have the means to do so and not resent or regret the decision of getting him and bringing him home and finding out the hard way it wasn’t such a good thing. But on the other hand, it could all go well too. I like to advocate preparing for the worst and if things are better, great!
Anyway, I hope this helps you make the right decision for yourself and please let us know what you decide!