Sorry to hear that, but I must admit there are plenty of 18 yr old horses without those issues! I would not let weight be a deciding factor when looking at a horse for sale.
Skinny horses usually need the following things:
Ulcer medication, deworming, teeth floating, and in the majority of cases they just need some good feed.
I would guess the horse is older than 18. Strange to have laminitis in a skinny horse. I've never seen a laminitic horse that was skinny, or are his feet just been that badly neglected?
If he is skinny and his feet are neglected, I would have no problem reporting the owners for abuse. It doesn't look like that is very nice quality hay either.