Gonna try and stay away from 2007 year model because of the spark plug issue. Question on that; What exactly is the issue with the spark plugs?
This is what happens when you read various opinions and take them as bible.
SOME, not all, Expeditions had "spark plug issues". Some owners and mechanics don't follow any of several techniques for replacement. They ratchet away like its a weedeater plug. The end result is "Ford is junk, had to helicoil, had to buy a new head, avoid this vehicle" mentality.
If you've found a 2007, 2008, or even a 2015 with over 100K miles and original spark plugs, you've found a truck where the basic, most routine maintenance hasn't been followed. Its probably got the original coolant in it as well. Should you avoid this? IMO not necessarily, you should negotiate the service(s) being completed before buying if there are no receipts in the glovebox indicating whats been done.
If at a dealership or private party: "Here is what I'll offer for the truck, here is a $500 refundable deposit check. Call me next Tuesday when all the work is complete and we'll finish the deal after another short test drive then." If they balk, you walk.
Also know that Ford sold like 90K 2007 models. 2008 saw just 55K. 2009 31K. So mathematically the 2007 is the most plentiful. Since its the most plentiful, it'll obviously have the most complaints, and keep in mind when it comes to reviews most people harp on negatives. Like 3rd owners who didn't check any maintenance history before buying-here, paying-here for a 2007 with 150K, after 3 plugs snapped when their brother in law (who took automotive in high school) tried to replace them one night after drinking.