I think I would just straight out ask them how much they are expecting for him. Start with a low price and if they seem hesitant or refuse, then you can go upward but if you offer a high price, you can't go back later and offer less. If your market is anything like ours, a pony that has been out of work for so long, is prone to laminitis, and is so obese would be worth relatively little even if he was well trained.
If you are serious about getting him, the first thing I would want is to have a vet thoroughly check him over. With him being so morbidly overweight, he is at great risk of insulin resistance and that may have been a contributing factor on his previous laminitic episode.
He sure looks like a sweetie, but you do need to consider that the upkeep on this pony may be much more expensive than you are expecting.