My 2006 F-150 5.4L supercab has 255,000 miles on it. Zero repairs. Just maintenance: tires, one belt, fluids, filters, batteries, brake pads. However, a spark plug change is $400+ and I had one done at 140,000 miles. The common knowledge is that the spark plugs can be easily broken upon removal by a ham fisted mechanic without the special tool. This is one maintenance job where you really should pay a dealers higher rate. Your chance of damage is virtually zero. A small private shop might have only done a few trucks. A dealer does them every day.

If you are buying from a dealer, get the spark plug change thrown into the price. If you are buying privately, your plugs are already 10 years old.