do you mean having a contract with leasing a horse?? Or buying?? Because is it pretty reasonable to to have a contract with a leased horse it generally just entails that the leasee is responsible for feeding, sheltering, vet checks and in the case of death a price which was agreed on before the horse went to the leasee will be paid by the leasee to the horses owner. When leasing it is pretty compulsary to stay in touch with owners.
But with buying normally the choice is yours I always welcome the people to inquire about the horse which they have sold me. However some old owners take it a bit too far we bought a horse off a lady about an hours drive away and she use to come by ever week and feed the horse take him out of the paddock and move him to other paddocks she thought were more suitable for him. Obviuosly we told this lady politly she was no longer welcome on the property after she abused us.
But I think the choice is yours and you can always discuss the contract and change bits around