Hey Ron,
I'm not an attorney, so this is not legal advice, but, if I were you, I'd use an Option to Purchase Real Estate agreement. If you don't have one, feel free to give me a call at 817-300-1132.
Be sure to make it renewable every few years, with or without a small renewal fee, and make sure to get it notarized and officially recorded.
Other than that, I'd just casually touch base with the potential sellers from time to time to see if they need anything.
Thanks,
Robin