I would highly recommend getting xrays, flexions only tell you so much. We had a girl at our barn do a prepurchase on a jumper who passed his flexions/lameness exams with flying colors. When the vet saw his xrays there were so many issues going on that he could "fall apart at any moment" and flat out said "do NOT buy this horse", which I"ve never heard him say before. And I haven't seen that horse at any shows recently... I usually recommend at minimum hocks, stifles, and front feet. I don't think you need to get his feet xrayed because of the absess but you do want to see if there are any navicular changes, coffin bone issues, etc. I'd also ask his owners if they've xrayed him before and request to see them if they have. It's always nice to have the option of comparing previous xrays to see if there are changes and to what degree.