What would you be leasing the land for? You would have to subtract any amount he could get for leasing to someone else (for hunting, Ag, etc), from the amount you are willing to lease for to look at what he might consider towards an eventual purchase. If he is smart, he would reserve the right to accept the highest purchase offer from you or someone else. As he knows you are hooked, he can keep raising the lease price every year. What happens if you break the lease, or he raises the lease amount higher than you want to pay, do you lose everything? So in short, you will pay an above average lease premium with very little security on your pymts against purchase. You would need a pretty well written legal contract prepared by an attorney to protect your agreement. Even then, if he defaults on the agreement your legal fees could exceed whatever you want to recover. How old is he, does he have children? If he dies this really through a wrench if they contest the lease by saying you "coerced" him into setting up a bad deal. Trust me, I had a friend who went though scenario, cost him $45k in legal fees and lost the deal in the end.
Lease to own deals are always set-up so that you pay way more for the item that if you were originally to purchase out right.
I would save your money for the down payment amount, you'll get there faster by not leasing. Get your financing set-up so you can offer him a cash deal with little of no contingencies. You could also consider a land contract. You could lease the farming rights back to him. Depending on the lease pymt amount, you could set the farm up as an LLC and use for write-offs of your equipment.