In your price range, knowing the service history is going to become rather important to have some confidence the truck will be reliable. Have you heard the saying, "you get what you pay for?"
My vote would be as new of an Explorer with the V8 as you can get for the price. Set aside a budget for maintenance items and transmission service. One nice thing about Explorers is Ford sold a lot of them in each of the model years so there is no shortage of parts and typically no shortage of folks who know how to work on and fix them.
-Rod