If the building had a great amount of value then it would likely have already been bought, converted and be occupied. Since it has not for apparently quite some time then keep that in mind. Buy the land. Use the building as a bonus since I think your probably not going to want to pay what it cost to go thought the demolition process and neither is the owner. How many people are going to be interested in the effort it will take to make the existing structure useful? Use that problem to your advantage. You can always offer more money later.
If you need to replace the plumbing then keep in mind you can perhaps place your appliance at the exterior edges and run the plumbing outside. Tie everything together outside. It’s pretty easy to run plumbing around a building if you have room to keep the grades as needed and it seems you have that. A whole bathroom or kitchen can run plumbing along a furred out section of the exterior wall of a room and then run water supply in and drain out one hole through the floor and footing. Interior framed up walls can carry plumbing to the furred out sections of the exterior walls. With a little thought you can likely take care of the plumbing needs with very little floor demolition work.