Buying land on contract is done all the time, you will have all the responsibilities of the landowner, i.e. paying taxes and so forth.
Here is what I would do:
1) Get a contract, there are many contracts out there on line some are free and some have a fee, but for $100 or so a lawyer will draw one up for you and really the landowner should split the cost because it is protection for both of you.
2) Then go to a Abstract company and get title work done, this is a must. If you do not get title work done you don't even know if the guy has clear title to the land. Not doing title work could expose you to a lot of risk. Title work will let you know who the owner is, that he has the right to sell it, if taxes are current, there is no liens or the property, and so forth. Cost is usually split half and half.
3) Go to a bank and set up an escrow account. This will have a small monthly fee, but the will keep track of the payments, give the seller his monthly installments, and keep the signed deed in a safekeeping vault. Once the loan is paid the deed would be given to you. Having an escrow set up will also eliminate conflicts arising from miscalculated interest, ect.