You could manage Aftermath stuff (Broxa mp5 or Kraken AK47) for about $180, that's bargain-basement but they're usable. Anything less then that isn't worth the money. What Kalnaren said is pretty much accurate other then that.
For "tactical gameplay in a woodland setting", you're going to need decent eye protection ($20 minimum), camo ($50 minimum usually, though if you keep an eye on the local surplus store you can sometimes get it cheaper), a vest or chest rig to carry mags ($40 minimum), mags ($40 minimum for a good loadout), some good ammo ($10 per 2000 rounds) and a decent charger so you don't fry your battery is a very good idea as well ($40 minimum).
So an absolute bare-bones kit for what you seem to want to do is going to run you about $400 per person. And to get some of those minimum prices I quoted you're going to have to hunt around for awhile for good deals, average prices for everything but the gun, ammo and charger are usually more like twice the minimum I listed.
As to power source, pretty much all airsoft rifles are battery powered, there are gas powered ones but they cost considerably more. As to what type of gun, I would recommend either an mp5, an ak47 or a m16/m4 due to avaliablity of replacement parts, upgrades and accessories.
As to where to find these guns. The best place is here on ASC, provided you're 18 years of age or older. If not then you're pretty much out of luck, there are a couple of websites that will sell you clear guns, even Aftermath stuff but the prices are going to be seriously inflated.