No, do not buy. A head gasket needing to be changed is often a whole can of worms more than just that... head gasket failure is often due to overheating, which means a warped block or head, with a slight chance of re-machining it back within specification, otherwise the engine is completely ruined. Throw another head gasket on a warped engine and it will still leak. If you can negotiate $8,000 subtracted from what the seller is asking, for a rebuilt engine, sure, go ahead and buy it.