Obviously older than you intend to buy, but I had a 99 F-150, 5.4. Loved it. 80,000 miles it started leaking oil. Found out is was a bad design. Ford issued a "Maintenance bulletin" for the repair. Then I found out a maintenance bulliten is what they do when they are trying to avoid a recall. In order to fix the oil leak, the cab had to be disconnected and raised up, in order to pull the motor. The estimate was $1,700.
I'm sure they fixed the problem by now as they may still have the 5.4, but they redesign all the time.
I still see the truck on the road. Probably still leaks oil.