It's a disorder that older horse sometimes get, I think it's a metabolic issue. I actually don't know that much about it, and don't have any really strong reason to think that he might have it. But, if an older horse is overweight, and if they tend to hang onto their winter coat longer than normal, grow a longer than typical coat and develop a 'cresty" neck, these are signs of Cushings. It can be managed with medication.
In buying an older horse I would really want to know a lot about it's health background. I guess I am a bit wary, why do they want to sell it?