The best bang for the buck? If you're going wheeling AT ALL, the first thing you do before you but a lift, tires, lockers, etc., is to make (or have made for you) a set of LOWER SHOCK MOUNT SKID PLATES for your rear shocks. Since I don't have a welder, I had a local 4x4 shop make some for me for $30. They have saved my rear shock mounts almost every time I have gone wheeling. There's an article on this site about it here:
http://www.explorer4x4.com/shockplate.html. The shock mount plates mentioned in the article are now obsolete from Ford, but steel plating for the job isn't hard to get.
Skid plates may not be as cool-looking as a lift & big tires, but you should start smart. You should also, as another cheap mod, extend your axle vent lines up high so they don't allow water into your diffs.
Next, I'd do a 2" lift. Front: either 2" lift coils or F-150 spacers. Rear: Warrior shackles and an add-a-leaf. You'll fit 31s fine, maybe 32s with after market wheels with less backspacing (stock is 4.5", go for about 3.75"). I got a good deal on some 33x9.50s and they fit after some trimming. I have the rear setup I just mentioned and Skyjacker 2" lift coils. To get it aligned after the front coild cost me $150 at the alignment shop because they had to add camber shims.
Then, but a lunchbox locker in the rear axle. I have a Lock Right and I love it. You could even get the locker before the lift and tires. The main thing is to learn how to drive off-raod and practice. Go out with some others who have done it before, then watch, learn, ask questions, and HAVE FUN!