I would suggest just keeping it simple and going with the Allen kit. You can go with a custom Kenne Bell setup like I did but it isn't exactly a walk in the park. The Kenne Bell will only be worth it if you plan to try to go for 400+ hp at the rear wheels. Not many people do that in their Explorers. You will be able to make around 340rwhp with the Allen kit which is still fairly respectable.
You will need a tune along with other supporting mods such as a new MAS and a larger set of injectors too. The Allen kit comes with all of that where as with the Kenne Bell, being that it is custom, you will be on your own.
Good luck, it gets expensive quick.