You cannot be sure it is the headgasket. It is only a good possibility. Removing the number one injector and checking for oil spray would verify the diagnosis about 90 percent of the time.

The owner may let you do this before purchase. Although you would have to be very careful how you arranged it. If he felt it was just a head gasket he might not sell it. Or boost the price.

Again if you are mentally prepared to find another engine there is a comfort zone in this deal or offer less. Being comforatable going in is important to future enjoyment of the car in my opinion.

Generally a head gasket change is a managable do it yourself item.