My father has me in his will to receive 57 acres & his house. I want to purchase adjoining property, only if I inherit his. How can I protect myself

You don't have any options. Your father has them. There are ways to help protect an estate plan against the possible results of a challenge and thereby better ensure that the desired beneficiary receives the intended assets. These could include your father creating a revocable trust and transferring the property to the Trustee of the trust before he dies, which could help avoid delay caused by a potential Will contest, giving you the property now or selling it to you (if doing so won't create problems for your father), or adding you as a joint owner now. But he should not take any steps without the help of an experienced estate planning attorney, and, since he is the owner of the property, HE has to be the one to take any such steps. You can't control the property right now, and there's nothing you can do to protect your expected inheritance except to try to ensure your father understands the potential value of doing good, solid, estate planning.