Depends what you want to do with him and make sure you get a vet to check him over even if it is just his knee before you buy it wouldn't put me off though
Depends on the injury and what you want to do, but it most certainly wouldn't put me off. My lovely ex-hurdler had twice slipped the tendon off his hock, which was enlarged as a result, and wouldn't have won him anything in the show ring, but as far as his soundness was concerned, he never had a day's lameness from it, nor did it ever affect his ability to do the happy hacker job I wanted him for. I was advised that jumping was probably not good idea, which suited me fine!!
The only thing it did affect was his veterinary insurance, which had an exclusion on all potential problems with that leg,
but it never caused any concern in the ten years I had him.