I had a COPD horse. He died last year at 27 years old. He was diagnosed when he was about 13 years old or so and lived a pretty long and useful life after that. In my experience/research the best thing for them is to be outside (away from dusty barns). They can be fed hay, but it needs to be not dusty. We would water ours. Another option is to feed cubes or pellets. It depends on how bad they are too. Red's medicine did a great job of controlling his symtoms when they got bad. We were definitely able to ride him. In fact, the vet encouraged us too and said it was good for his lungs. The only thing to watch for was not to ride him when he was having an attack. I've heard that the triggers can differ too. In Red's case, besides normal dust & such he had a really hard time when the corn fields were blooming (pollen in the air) and when it got really hot & muggy. This got much worse as he got older.
All in all, I wouldn't advise against buying the horse as long as you know how bad the symptoms are and are prepared to deal with them. However, IMO if you are looking for a competition horse I wouldn't get a COPD horse... but for a trail horse I think they would be fine.