If you have 100k miles on the original wires then it is definitely time for them to be replaced. Especially if you are having problems with misfires. Typically when wires go bad the spark can jump between wires or other things. One way a lot of people look for bad wires is to open up the hood when it is very dark and look for sparks.
A lot of people seem to be in agreement that Magnecore wires are the best. I think they are 8.5mm I think they run around $85. Not sure if they fit in the wire holders.
Ford Racing (FMS) wires can be had for $40-50. These are 9mm wires (stocks are 8mm)and altho they are a direct replacement they will not fit in the stock wire holders. Also some people have had issues with these wires going prematurely. I have them and have no problems.
If you let your mechanic do it with the stock OEM replacements you will be happy with the results. You will not be getting ripped off either. It's just you could go a different route and have it done cheaper. The choice is yours.
I live in Detroit, I couldn't give a **** how good my car is in the "twisties."
"I could use a hundred people who don't know there is such a word as impossible."
~Henry Ford
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