I agree with Fix4Dirt. The 2006-2010 three valve V8 Explorers and Mountaineers are reliable, get 20-21 mpg on the highway and 17 mpg mixed driving, tows 7,000 lbs., has side airbags, weighs nearly 5k lbs. so it holds its own in a typical accident, is easy to get in and out of, seats 6-7 people and they drive and perform nicely. If the engine and transmission in these have been well maintained they can last 200k-300k miles, or more. Try to find a 2009-2010 model as these received some updates to the V8 engines and a slightly stronger transmission.
Stay away from Explorers and Mountaineers with the 4.0L SOHC V6 engines. They have timing chain and guide issues that can destroy the engine and to replace all the chains and guides requires the engine to be pulled. Also, the V6s come with an unreliable transmission that can fail any time after 100k miles is reached. The typical cost of repair for these items is around $3,500 for the transmission and $4,000-$5,000 for a remanufactured engine. The two valve V8s in the 2002-2005 Explorers and Mountaineers are very reliable but they come with the same transmission as the 4.0L SOHC V6 so at least you are mostly limited to a transmission failure in these models.