Until you see the whole picture clearly, you're not in a position to determine whether you have a major problem to deal with or not. What happened to the Blacks is typical of what can happen over time with older homes. The drainage work that was completed years ago was probably adequate at the time. But since then, there had been unprecedented rains in the area, which caused flooding in many basements. Also, in the intervening years, drainage technology advanced.
New technology can be more expensive but often does a better job.
The Blacks considered calling in other drainage experts to see if the work could be done for less. However, after studying the buyers' inspection report, the contractor's proposal and the buyers' offer to split the cost of the drainage work 50-50 with the sellers, the Blacks concluded that they had a fair deal. The solution is not always this easy, especially when contractors can't agree. Keep in mind that there is an element of subjectivity involved in the inspection process. A second roofer disagreed.