My hubby is a Ford guy, so I cannot speak for anything else. We love our Fords, even the "dreaded" 6.0 liter. My hubby is a mechanic before becoming a farmer, so weve bought an 04 and 07 that had engine problems when we bought them. We fixed them up, and sold the 04 after 7 years of enjoyment. The 07 F550 was a company owned rig that had been abused. We've done the motor, transmission and transfer case, but we're sure it's all good now.
My hubby said to stay far away from the 6.4L. They were worse than the 6.0.
Now for the good.... we LOVE any ford truck with the 7.3L available in older fords. Our 96 has over 300,000 miles and we haven't had any major problems in the last 125,000 miles we've had it. If you want reliability you can't go wrong for with one with a 7.3L, which stopped production in mid-03.
Bottom line, if you find an EARLY 2003 Ford or older in good shape don't worry if it has higher miles. Check the engine size under the hood. If you really want a Ford 2003 or newer, have it checked out by a GOOD mechanic skilled in FORD TRUCKS. Many regular mechanics do not know the nuances of the 6.0, 6.4 or 6.7. Good luck in your search!