With what I've experienced on '03 Tecs' is just figure on replacing the coil pack every 70,000 - 80,000 miles. About $140 for a Ford Pack currently and an hours work to install. One hint. Replace the plugs, plug wires and PCV while you have things apart!
My '96 coil pack is the original one. 173k miles. Just spark plugs need to be replaced at 80k miles. Wires??? Only if they look burned on outside.
The only drawback is that the spark plugs need to be at least double platinum. Ford startad with single platinum on a bank (positive spark) and double platinum on the other (negative spark). Was confusing for users so they went duals both banks. Then, they went to fine-tip platinum (just short of iridium fine-tip).
Coil-on-pack? Spark plugs can be less expensive (regular platinum), but the coils in the back really take a beating from the heat. Don't know, maybe they had reability issues with the first models, that's why they went back in 2003(?) to coil packs...
96-99 single coil, 00 coil on plug, 01-03 single coil, 04-05 coil on plug (Duratec 3.0s in the Fusion, Five Hundred, etc all have used coil on plug). I'm wondering if Ford wanted to stick to the coil on plug design on the 04+s since the newer cars in the product line were using this design?
Coil on plug is better for the dealer/repair shop. Coil on plug assemblies will go for $70-130 EACH. A coil pack will go for $80, wires for maybe $80?