As I see it you do not need contract,
What you must do is both parties go to the land office and transfer your name to the chanote.
I describe the simple circumstance assuming that you are Thai national..
Given that you are not a Thai national-then you will need to set up an accountant such that a Thai company can be set up.
If the advise is such that this is not possible-then the deal is off.
However the current owner can lease the property for max 30 years.
you will typically pay the full price -for the lease.
After 30 years the control reverts to the official owner.