if you go with a roots style supercharger, installation would be as easy as a mustang alternator(offset different than the explorers), essentialy a manifold swap(easy as well...i can get mine off in about 30 minutes), and a custom intake pipe...its all about the money, good bets for the gen 3 v8 explorers are the Ken bels, and the saleen superchargers...there is currently NO KIT MADE TO FIT THE EXPLORER...there will be some custom work involved...nothing to serious though....as i tend to warn everyone...these engines cylenderheads do not like to deal with above factory power specs...just this last year with only full bolt ons and roughly 300 at the wheels...ive shot 3 spark plugs...so if your supercharging you might want to invest in some aftermarket heads w/ anti-blow out inserts...also, the 5r55w does not like to hold more than 400 for long...or so i have been told.
if i had to guess, id say the price of supercharging an explorer(a full upgrade w/ injectors, fuel pump, fuel pump computer, tune, alternator, supercharger)would be around $6500-7250...and that is not including labor...there are also mods that need to be done before(mainly free flowing exhaust(headers and a catback system) and at least a K&N intake prefferably with at least a 70mm throttle body).
i plan on installing my supercharger this summer...im going with a kenne bell...its going to be awesome ^.^