If there are significant changes already and the horse is only 8 years old, I personally would pass on him; a horse this young has an awful long time to live yet, especially if he is in pain already on the flexion level.
He may or may not be able to perform for you for years to come. And don't let the owner fool you into thinking he can. It will highly depend on the horse.
Yes, there is always joint injections, and supplements, and other pain remedies, but these are far from fool proof, and may only improve his joint pain somewhat, and may not help it for 'long term'; ie, he still may not perform for a long time, even on all of those options.