Ask what his behaviour is like ridden and on the ground, ask if he has any vices, and what he has done in his ridden career to date. Bear in mind his age...
I would ask if they have any history with him from previous owners unless he is homebred, I would also ask what position he assumes in a herd and wether or not he can live in a mixed herd..
I would also ask what he is like with traffic and farm machinery, with the farrier, in a stable.
What feed is he fed and supplements if any...
Does he load...(check don't just take yes as a definate answer)
What tack and bit is he ridden in and why..
I would want to see him caught from the field, tacked up and ridden before I tried him. Also wether he is nervous when on his own and if he will ride out alone.
I would be specifically looking for any behavioural issues when ridden such as rearing, bucking, bolting etc.
Those are just a few of the things I would be asking...I would also ask if I could have a trial period (not sure if you do this in the you.S but we do over here)..x