I've looked at 4.8is's with your kind of mileage. Even ones with good maintenance records. At that mileage the valve stems seals are about to take a dump. If you don't do it yourself you're looking at a $5,000 repair job. On a $10,000 vehicle. Too rich for my blood. Not to mention timing chain guides. I haven't read your add, but I didn't see anyone mention if those have been done, but they're getting close as well. Another very expensive repair. As is the cooling system, including the crossover pipe. That's getting ready to go as well if it hasn't been replaced. I think that is why you're having trouble and that's why when someone finds one for $5k they think they've hit the jackpot. They have if they are able to take care of those things themselves. Good luck!