The question most likely hinges on whether the repair can be made without removing the head. That will not be able to be determined without making an attempt first. Or, at least seeing the problem.
My approach when buying a car with system deficiencies the owner has felt best left with the car, is to place the highest possible value on the possible repair and deduct it from the reasonable purchase price.
Considering the possibility of head removal I would adjust the price by atleast 20 hours labor for one head. A professional should view the problem to be sure someone hasn't screwed it so bad that a different head would be needed or that the manifold isn't so warped that it too would be required.
The best arrangement is for the seller to have the problem fixed by a mutually acceptable professional. This removes the risk and allows the least amount of money to be removed from the selling price.
Bosch Master, ASE Master, L1
33 years MB technician