Why not go to an established used car dealer in your area? Does the appraisal have to meet specific criteria? If not, something on dealer letterhead indicating the specifics of the vehicle (year, make, model, color, equipment, mileage and condition) with a valuation should cover it. Ask Carmax if the estate executor (with papers from the county Register of Wills confirming his status as executor) could stand in for the owner.

Does the estate have an attorney? What is his advice?