Assuming the seller is correct in saying that this is a V6 with manual transmission (and assuming no PO put a different engine in it), it will be the correct engine. There was only one V6 available on the Mustang II. You will still need to get a handful of Ranger/Bronco parts, but the MII parts are the most difficult bits to find.
Of course, the engine could be junk, in which case you may end up paying a fair amount for what your really need (bellhousing, timing cover, flywheel, pulleys, etc.). But there are some desirable parts on the MIIs (I believe hot rodders value the front suspensions and the 8" rear ends are good for more than Alpines), so you could probably recoup at least some of your investment by parting it out. But, IMHO you should be careful paying too much for any car that includes this statement: "I was told the car would run, but the gas tank is in the rear hatch and not hooked up. So I don't know.". Still, it is tempting...
