Fury-
I am not sure how to help you out here. I don't know anything about you (your Bio is not filled out) so I don't even know if you are legally able to purchase and own airsoft guns.
You can't compare airsoft to paintball directly, but I'll try to answer your questions. If you use quality paint and tumble the balls the night before your games, high end paintguns are extrememly accurate and have great range. Most paintballers don't bother to aim, though. A paintball hurts a heck of a lot more than an airsoft BB because of the mass involved, even though it is dispersed on impact. The welts are much larger in paintball and the effects last longer.
Airsoft is much more demanding than paintball in that the player is required to get closer to the target because of the lessened range. It encourages skill over firepower.
There are no local stores worth looking into. Prices are outrageous and selection is horrible. Online shopping from Hong Kong is the only alternative. Age restrictions are the law, though.
Your choice of an M4 is a good one. It's a great airsoft gun and there are a million accessories for it, from the basics, like mag pouches, to the popular "must have forty items on my gun" rail items. Tokyo Marui is a known company and their guns are great. Classic Army makes an all metal version that met with some problems right off, but a few guys here have them now and can respond to the quality of their guns better than I can. The metal body adds a lot of realism and strength, but is not "necessary" as much as it is a nice addition to have.
Overall range on airsoft guns, the electrics, usually does not exceed 100 feet, and that is a long shot. Gusn can shoot farther, but the BBs are so small and lightweight that accuracy beyond that, and sometime out to that range, is poor. You can learn to adjust for it, but that's about it.
Let us know a little more about yourself and it will be easier to help you, but maybe this can give you a start.
"You are, what you do, when it counts."
-The Masao
"Get on that fifty! Someone get on that fifty!"