Replacing plugs and wires isn't too bad. I have never gone through the fenderwells but I hear it's easier. Pulling the boots off is the only part I hate because when they come off my knuckles bounce off metal parts and leave skin behind. The wires are easy to replace, there's plenty of room to reach around the back of the intake and unclip them. Make sure when you put the new ones in to keep them routed through the same clips and off of hot components.
If replacing plugs and wires at the same time, a few tips.
Replace one plug and one wire at a time so you don't mix up the order. For best results gap the plugs properly (I think .056 to .062, but check the sticker near your fan to be sure), then put some anti-seize compound on the threads before screwing them in. You can use a short piece of rubber hose to thread them in hard to reach areas - slip it over the tip and thread plugs in with it, it won't strip the metal that way if you crossthread. Run the plugs in snug but don't reef on them, too much torque can break things. Put silicone dielectric compound on the metal tips. Push the boot all they way on, wiggle it and tug on it gently to be sure it's seated.
Don't use the Bosch plugs, they don't provide any performance gains (I have tried them twice) and are unnecessarily expensive... try Motorcraft platinums or even Autolites. For wires, the Bosch premiums are not that great either, I had a brand new one where the connector was loose on the wire inside the boot and it made poor contact.
Good luck!
