Well sir, I am an Insurance Adjuster
my estimate is to repair the vehicle perfect using OEM parts. Even if the headlamp is working and not broken if there is any cosmetic (scratch, chip, nick ect) it would be replaced. The blend on the Lt Ft door is to insure that the paint all matches. Otherwise you would be able to tell that just the fender had been painted.
However there are many ways to repair a vehicle, your plan of just buying a fender and putting it on would be the cheapest way to do.