I would start the first week just feeding and handling him but no riding. The second week would be lunging and ground work. By the third week I would have him under saddle but would wait until then or even the 4th week before riding. When I did ride him it would be very light work for a solid month, increasing it each week until I had him out for a couple of hours. I would not be jumping this horse for a while. He needs to come back slowly.
Many times a horse in that condition will be very lay back but as soon as you build them up they are a different horse altogether - just something to keep in mind.
You mentioned that a possible trade was involved. What is that about? How much do they want for the one you are looking at.