I know this is months old but I have to make sure I go this right since I am looking to buy a 4th Gen (now looking at a Mountaineer Premier V8).
$800 to replace the spark plugs and $3200 to fix the exhaust manifold studs (both sides)? Did I get that right? And I am guessing (hoping) that is the cost to have an auto shop do the job instead of DYI.
How about DYI?
That individual was greatly exaggerating.
Regarding the plugs, now that more users are following one of the proper procedures, we're seeing far fewer issues here for DIY. There's also a spark plug removal tool available now for this model should you mess up and break one. I just did mine 1 month ago, soaked the living daylights out of them, used a torque wrench set to what the TSB recommended, and out they came. Not a single broken plug (thank goodness.) Truck had 90k on it at the time. Only cost was the cost for 8 plugs, nickel anti-seize, a torque wrench (which I didn't own before,) and a special long-reach spark plug socket made for these 4.6 and 5.4 engines.
Now moving to the exhaust manifold. That indeed is a pain. I'm in the process of getting mine done now. BUT - if you do need to do it, upgrade to stainless bolts. Solves the issue. Also it's NOT $3,200. Give me a break. Highest I've ever seen anywhere is $1,100 per side. Many people get them done in the $400-$600 per side range. I was quoted $600 today myself.
Also, there are now tools available for this as well to make the job much easier especially if you're DIY:
http://www.promaxxtool.com/rocky.html
Those are the only two 'major' issues I'm aware of for the v8. Neither of which in my mind are show-stoppers. I've had ours for 5 years and we've liked it. Radiator leaked but that's also an issue on the v6 (they use the same radiator.) Had it replaced by a friend - wasn't that bad. Didn't happen until just last month (did the plugs at the same time.) So it lasts a decent while imo.
As far as the v8 vs v6 goes, one thing we appreciate about the v8 is just how much more smooth it is overall. That 6 speed is also silky smooth. Ours is a 4wd so mpg is in the toilet, but we do appreciate the extra power for towing.
Like others stated, get it with the tow package. It took us a LONG time to find one like that. It was like 95% of the v8's out there had the stupid tiny receiver and not the 'proper' size (i.e.: 'man size') lol.
If you're shopping for a v8. When you go to test drive it, make SURE it's bone cold when you first start it up. Then once you start it, listen for a fairly audible "tick tick tick" (don't confuse the sound with the injector ticking, which is normal.) Again you want to ensure it's cold because the sound will go away in about 90 seconds if it does indeed have an exhaust manifold crack/broken studs.
Also as a general tip. Ask the owner when he last replaced the fuel filter. Many owners of the explorer start to complain about hesitation/bucking/ etc at around the 90k+ mark. You'll hunt it down anything from coils to plugs to the transmission. Many times though it's the fuel filter - a commonly neglected item and, for whatever reason, the explorer seems to be particularly picky about having a clean one.