1999 Ford F-150 P0420, P0401, P0306 Codes: Cost-Effective Diagnosis and Repair Guide

Hello everyone! This is my son's first truck, and I recently pulled three OBD-II diagnostic codes: P0420, P0401, and P0306. While I understand what each code means, I'm looking for a clear, cost-effective path to fix these issues. The P0306 code indicates a misfire in cylinder 6. Although no defective coil fault code is present, could this still be due to a failing coil pack? I’ve heard that misfires can sometimes appear without a specific coil failure code—should I still replace the coil pack as a first step? I’m also wondering if P0401 (EGR flow issue) and P0420 (catalytic converter efficiency) are related. The truck has 179,000 miles on it, and I’ve noticed a faint rotten odor from the exhaust, along with noticeable surging and bucking under load. I plan to swap out all coil packs and clear the codes to see if any new misfires appear on other cylinders. However, could these symptoms be caused by a faulty DPFE sensor, a clogged EGR valve, or a defective EGR solenoid? I’m trying to minimize repair costs while ensuring reliability—especially since my son will be turning 16 in two months and needs the truck to run smoothly. Any advice on prioritizing repairs would be greatly appreciated. Thanks for your help!