I would also see if the parents are on site and to take a real good look at them. Seeing the grandparents would be even better. Look at their fetlocks and pasterns to see if there is any swelling, even a small amount of swelling like windpuffs could be the beginning signs of dsld. Definately a flexion test and ultra sound, but the only problem with this is that even if a horse has dsld and is not affected by it yet the test results can come up negative and the horse could become affected later on and test positive. So, if you can see the parents soundness and take a good look at their fetlocks and legs to note any swelling or dropping of their pasterns that would be a more acurate proof of negative for dsld. I'm looking right now at a gelding whose lines have only been in the States for two generations and this gelding's father who is now thirty years old is still well built and sound that the owners grandchildren still ride him!