How to secure a property when it's not yet for sale

[JT Spangler]

So, I bought a house from a nice older couple. It was their mother's house, and they live next door. The two lots are totally fenced in together with no fence between them, and in total they're 1 acre (I have 0.6, and they have 0.4). 

My long term exit strategy is to acquire their house (for the lot) and sell the 1 acre to a developer. If East Nashville keeps building like it has been, this could net me 200 to 500 k, minimum, which I'd 1031 into a large multifamily. Or so the thought goes.

My question is: what's the best way to secure the right to buy their house when I know they have no intention of selling any time soon? They spent some of the profits on the earlier sale to me on upgrades for their house (they're both retired and on a fixed income, I assume), so I doubt they're planning to sell. But they're probably in their 70s, so at some point I assume they'll need to go elsewhere. I just want to make sure that when that happens, I get the right of refusal on their house/lot.

Could I draft some sort of option contract where I pay them some nominal sum now for the right to try and purchase the house when they get ready to sell? If so, how do I insure that they don't try and gouge me when the time comes? I don't mind paying fair value, but don't want to get hosed. And the property is worth more to me than anyone else, since controlling both lots is the way to maximize values.