As someone who has a 2V 4.6l I've tried just about everything from bolt-ons to nitrous to SuperChargers.. Only thing I haven't done is a turbo setup and by far the best bang for your buck is nitrous.. With that being said, there is the hassle of filling up the bottle etc and probably the least safe of all power adders if it's not done right.. No matter what, you're going to loose some MPG as it's the simple fact that it takes more fuel to make more power..

What I would suggest doing if you're on a budget is look for a used S/C kit on corral.net or modularfords.com.. There are several on there that you can find for a good deal.. I would personally go with Vortech myself, I went with Paxton and even though I've had no issues whatsoever with it, finding parts such as pulley's can be a pain in the butt compared to the Vortech.. Kenne Bell's are very nice but you're going to pay for them and if you want to leave some fuel economy in your car I would avoid them as their power curve is completely different than a centrifugal supercharger..

Good luck and have fun!