Being swaybacked isn't the end of the world - my friend had a 15hh palomino with a swayback, had him until he was 37 years old. He taught a ridiculous number of kids to ride before he retired from the riding school life at 25, and did pony club from the age of 7 or 8 right through to 30. He could jump up to about 3 ft, and whilst he couldn't go higher up to that he was steady as they come and gave the kids real confidence, and he was fantastic cross country, would win every time. He was still being lightly hacked up until the summer he got PTS at 37 - and still would get strong and want to canter!
He was lucky though in that he was never asked to carry anyone heavy, and that's probably what kept him going so well all his life. The swayback was only ever an issue in fitting his saddle, and whilst he would never have reached the stage of trying to collect as he couldn''t effectively use his back, he coped really well around his problem. He finally got PTS when he had no teeth left and the winters were too hard on him. So don't discount a swayback from being a great riding horse, just make sure you work around their issues and don;t expect too much of them. If you don't expect too much, you'll be pleasantly surprised!