The 5.4 is a good engine and there are a lot of E-Series out there with 400-500k miles and counting. It did have one major issue around that time and that was the spark plugs and the threads for the plugs on the heads. The spark plugs would seize up in the heads and when it was time to change them they were easily broken off making an easy job into a very big ordeal. Ford came out with special plugs for those engines, a tool to assist in removing the plugs and I believe they now recommend the plugs be changed every 25k miles. It's not uncommon to see 5.4's from that time frame where the heads have been helicoiled as the original heads were often damaged when changing the plugs.
I have 155K miles on my 2004 5.4 E-Series and have not had any problems with the plugs. I make sure and get the engine hot prior to changing them, use compressed air to blow out the plug area , soak them down with penetrating oil and apply a small amount of high temp anti seize on the outer edge of the new plugs.
