Carl, many years ago the rule of thumb was that the tow vehicle should weigh at least as much as the trailer. Since an mpg has a gross weight of 3800 pounds, you want a vehicle that weighs about that much. That is most modern vehicles, I believe. Back then you were also supposed to have a weight-to-horsepower ratio of less than 60:1. That means that you need at least 125 horsepower in your tow vehicle. Again, most will be far stronger than that.
As for what to pick for a tow vehicle, I'm not going to tell you what to buy because I don't know your financial situation, how many people will be riding in the vehicle, what else you will need it to do, etc. I will say that our Mountaineer tows our mpg just fine. The Explorer is the same vehicle.
To figure actual towing capacity, you will need several numbers. The dealership should be able to tell you the empty weight and gross weight of your vehicle. As Marc said, you will need to add the weight of the fuel (6 lb/gal), passengers, and whatever other stuff you normally have in the car. Then add the tongue weight of the mpg (use 350-400 lbs). The sum of all of that should be less than the gross weight of your tow vehicle. The dealer should also be able to tell you the Gross Combined Weight Rating of your tow vehicle. Subtract the weight of your tow vehicle all ready to go from the GCWR and you will have the weight of the trailer you can tow. Since you are looking at an mpg, you want that number to be at least 3800 pounds.