I would buy it.
Certainly try and negotiate because it wouldn't hurt. You have nothing to lose.
The full price is certainly a fair price anyway. You will likely never find another E320 with that low mileage and a new head gasket.
The head gasket repair alone is about a $1,500 job.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".