When contemplating the purchase of any property, a standard home inspection by a licensed home inspector is always in order. In the case of antique buildings, I also suggest an evaluation done by a contractor and/or architect who specializes in antique structures. I particularly like the feedback of experienced contractors who can offer ballpark figures related to costs.
I think it is important to consult with someone who will not ultimately be bidding on the work. The evaluation should not be colored by a desire to be awarded the contract. Perhaps someone from outside the immediate area is best. I know of a contractor with 40 years experience in old house restoration who will look at a structure and give his professional assessment for an hourly rate of $65.
This preliminary feedback is enormously valuable to the buyer in determining whether or not the subject property is appropriate for their level of financial and emotional resources.