Sounds pretty bad. I don't see the use of going through the time and effort of a diagnosis for the seller. The best approach for you is to assume that the engine is a total loss at this point. It could be a cracked head, massive fuel dilution, engine starting to seize, head gasket, bad oil pump, worn bearings---who knows? Why would you want this vehicle when there are so many good running Troopers to be had at dirt cheap prices?
if you want this particular vehicle, buy it at junkyard prices, and then shop for a used engine. Maybe you'll get lucky and find the engine isn't totally ruined, but I'd assume the worst, given your symptoms. What someone needs to do is a cooling system presssure test, a cylinder leakdown test, a test for combusion gases in the coolant, and to drop the oil pan and investigate.