It's easy to think that having a 'bond' means that the horse will be perfect. I guess I don't subscribe to the idea of a magical bond at all - while it's true that certain horses get on better with certain people, a true bond in the most realistic sense of the word is formed of respect first and then trust, and this takes time to develop. Be prepared that with a horse with disability, it will take longer, and you will have different challenges.
As to how rare his behavior is, it depends on his individual personality. You'll have to ask his owners.
That being said, many horses with blindness in one eye function perfectly fine, even becoming successful showjumpers - what do you plan on doing with him? Many people opt to have the eye removed as partial vision on one side can cause spooking. You may find that this is partly what causes his anxiety. A horse will adapt faster to having one eye than having one eye that works and one that is cloudy. If you decide to go with the horse, get a vet PPE. It may be that the condition will eventually effect the other eye and you'll be stuck with a totally blind horse.
My gut instinct would say that this isn't the right horse for a first time owner. But, if you have enough support and time, and get a PPE before buying, you could make this work. Just don't let the horse's initial friendliness cloud your judgement.
Edited to add: I just noticed that you said his condition changed overnight. If this is true, I can't stress enough how important the PPE is (and to be honest the idea of a deteriorating condition is enough to put me off).